Marine seismic survey method and system

ABSTRACT

An inventive method provides for control of a seismic survey spread while conducting a seismic survey, the spread having a vessel, a plurality of spread control elements, a plurality of navigation nodes, and a plurality of sources and receivers. The method includes the step of collecting input data, including navigation data for the navigation nodes, operating states from sensors associated with the spread control elements, environmental data for the survey, and survey design data. The positions of the sources and receivers are estimated using the navigation data, the operating states, and the environmental data. Optimum tracks for the sources and receivers are determined using the position estimates and a portion of the input data that includes at least the survey design data. Drive commands are calculated for at least two of the spread control elements using the determined optimum tracks. The inventive method is complemented by an inventive system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/598,732 filed Sep. 8, 2006, which is a U.S. National Stageapplication under 35 U.S.C. §371 of International Application No.PCT/US2004/008029 filed Mar. 17, 2004; both of which are incorporatedherein by reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the performance of a marineseismic acquisition survey, and, more particularly, to the control ofthe seismic survey spread during the survey.

2. Background of the Related Art

The performance of a marine seismic acquisition survey typicallyinvolves one or more vessels towing at least one seismic streamerthrough a body of water believed to overlie one or morehydrocarbon-bearing formations. In order to perform a 3-D marine seismicacquisition survey, an array of marine seismic streamers, each typicallyseveral thousand meters long and containing a large number ofhydrophones and associated electronic equipment distributed along itslength, is towed at about 5 knots behind a seismic survey vessel. Thevessel also tows one or more seismic sources suitable for use in water,typically air guns. Acoustic signals, or “shots,” produced by theseismic sources are directed down through the water into the earthbeneath, where they are reflected from the various strata. The reflectedsignals are received by the hydrophones carried in the streamers,digitized, and then transmitted to the seismic survey vessel where thedigitized signals are recorded and at least partially processed with theultimate aim of building up a representation of the earth strata in thearea being surveyed.

Often two or more sets of seismic data signals are obtained from thesame subsurface area. These sets of seismic data signals may beobtained, for instance, by conducting two or more seismic surveys overthe same subsurface area at different times, typically with time lapsesbetween the seismic surveys varying between a few months and a fewyears. In some cases, the seismic data signals will be acquired tomonitor changes in subsurface reservoirs caused by the production ofhydrocarbons. The acquisition and processing of time-lapsed threedimensional seismic data signals over a particular subsurface area(commonly referred to in the industry as “4-D” seismic data) has emergedin recent years as an important new seismic prospecting methodology.

It is common practice for a certain amount of information about thesurvey area to be gathered beforehand so that the appropriate equipmentand methods can be selected (known as the “survey design”) to achievethe desired geophysical and operational objectives. Some of thisinformation is used to provide the basic parameters for the survey, suchas the boundaries of the survey area, the lengths of the towed streamercables, and the firing of the seismic sources. Such information has, tosome extent, been used to assist in survey control through variousindependent systems. Typical of such control systems have been vesselautopilots, ship heading control, and towed cable positioning and depthadjustment. For example, U.S. Pat. No. 6,629,037 describes the use ofcost maps to optimize paths for seismic in-fill shooting within a knownsurvey area. British Patent Application No. GB 2,364,388 discloses thepositioning of seismic sources and streamers within a known survey areaaccording to recorded position data from a prior survey.

It is also well known for a certain amount of information about thesurvey execution to be gathered during the survey (i.e., in real time ornear-real time) so that the appropriate settings and positions can beachieved according to the desired geophysical and operationalobjectives. Such information has, to some extent, also been used toprovide survey control through various independent systems. The state ofthe art in such control systems is represented by the following patentreferences: U.S. Pat. No. 6,618,321 (simulation of streamer positioningduring a survey according to current determination); U.S. Pat. No.6,590,831 (coordination of multiple seismic acquisition vessels during asurvey according to monitored survey parameters); U.S. Pat. No.6,418,378 (neural network trained by survey-acquired data for predictingseismic streamer shape during a subsequent survey); U.S. Pat. No.5,790,472 (positioning of seismic streamers during a survey according tohydrophone noise levels); and International Patent Application No. WO00/20895 (seismic streamer positioning during a survey according toestimated velocity of streamer positioning devices).

The control systems described above rely upon particular inputs (e.g.,marine current) to determine information (e.g., passive streamer shape)useful in controlling a seismic survey towing vessel. None of thesesystems, however, relies upon or takes into account a broad spectrum ofinput conditions and parameters that includes the various objectives andconstraints of the seismic survey equipment and methods. Furthermore,none of these systems seeks to actively control the spread with acoordinated suite of steering devices deployed throughout the spread. Aneed therefore exists for such a comprehensive system.

The control systems mentioned above have been designed to achievedesired results by providing outputs, such as commands or paths, forimmediate implementation. There has been little or no consideration insuch optimization of the important time-delayed effects of theseoutputs. A need therefore exists for a seismic survey control systemthat accounts for time-delayed effects of outputs—particularly controlcommands—as well as the immediate effects.

DEFINITIONS

Certain terms are defined throughout this description as they are firstused, while certain other terms used in this description are definedbelow:

“Angle of Attack” is the angle of a wing or deflector relative to thefluid (i.e., water) flow direction. The angle of attack is a derivedquantity, computed from the orientation of the deflector or the body towhich the wing is attached in the system reference frame, thecontrollable or fixed orientation of the wing relative to thedeflector/body, and the direction of the current in the system referenceframe. When the wing/deflector has no lift, it has zero angle of attack.

“Area rotation” means an axis rotation from the north orientated axis.Thus, e.g., a 0° area rotation means the shooting direction, ordirection of tow, is north. This gives the area-relative axes'orientation and determines the shooting directions for the survey.

“Base survey” means the original survey, and associated spreadcoordinates, that a time-lapse survey is trying to repeat.

“Course made good” means the actual track made with respect to theseabed.

“Cross-line” and “inline” mean perpendicular and parallel (respectively)to a direction of tow, and are defined in an area-relative referenceframe. The reference frame origin may be translated to the vessel. Anexample of the inline axis orientation is parallel to the pre-surveydesignated shooting direction, (e.g., pre-plot line direction or arearotation).

“Drive commands” means changes in the spread control element operatingstates that will give a desired outcome in the positions of the spread.

“Force model” means a computer-implemented representation of the impactof a significant set of hydrodynamic forces on the spread. The forcemodel includes representations of the spread and the medium (i.e., thesea and atmosphere) in which it functions. This medium includes verticalregion from less than 40 meters below the sea surface and some 10s ofmeters above the air/sea interface. Forces generated outside thisdefined zone but that have resultants in this zone are also candidatesfor modeling.

“Natural Feather” means the angle between a line defined by any twopoints on a towed body and a reference direction, commonly the vesselshooting direction, where the points get their position due to theeffect of current, wind or both. An example is the angle between thestraight line formed by connecting the front and tail of a streamercable and a pre-plot line direction.

“Near-real-time” means dataflow that has been delayed in some way, suchas to allow the calculation of results using symmetrical filters.Typically, decisions made with this type of dataflow are for theenhancement of real-time decisions. Both real-time and near-real-timedataflows are used immediately after they are received by the nextprocess in the decision line.

“Position history” means coordinate or shape estimates at discrete timesfor any spread element or group of elements making up a spread component(e.g., a streamer or source array). Two coordinate or shape estimatesmade at discrete times gives an average velocity over the timedifference. Three coordinate or shape estimates at three, differenttimes gives two average velocities, and one average acceleration.

“PID” or “PID Controller” means a Proportional-Integral-Derivativecontroller, a type of feedback controller whose output, a controlvariable (CV), is generally based on the error between some user-definedset point (SP) and some measured process variable (PV).

“Predicted residual” means the difference between spread model positioncoordinate predictions and independently determined, navigation-basedposition coordinates. This term is borrowed from Kalman filterestimation theory.

“Present survey” means raw data collection, computation results oractions that have been generated in the course of the survey presentlyundertaken. These may be used in real-time, near-real-time or asotherwise required.

“Prior survey history” means any data that is used in the preparationfor, or execution of, the present survey, which was generated before thepresent survey began. Examples include a base survey, maritime charts,tidal information, depth information, seismic maps, borehole data,binning data, and historical records of natural feather. Suchinformation may or may not be in the public domain. This data may beobtained during a preliminary survey.

“Real-time” means dataflow that occurs without any delay added beyondthe minimum required for generation of the dataflow components. Itimplies that there is no major gap between the storage of information inthe dataflow and the retrieval of that information. There is preferablya further requirement that the dataflow components are generatedsufficiently rapidly to allow control decisions using them to be madesufficiently early to be effective.

“Shot points” means the unit of time corresponding to the temporalseparation between seismic data acquisition events.

“Shot point target coordinates” means the intended two-dimensionalcoordinates for all spread objects to occupy in order to collect seismicdata. This set of coordinates can be used to derive a spread body targetshape as well.

“Spread” means the total number of “spread components,” i.e., vessels,vehicles, and towed objects including cables, that are used together toconduct a marine seismic acquisition survey.

“Spread body shape” is a mathematical function describing the shape ofany of the towed spread components. As an example, a streamer cable maybe assumed to have a straight line shape from end to end. Alternatively,the shape may be a series of lines or higher order polynomials,connected between an arbitrary set of position coordinate estimatesalong the streamer, to give an approximation of the shape of the totalstreamer. A similar method can be applied to the seismic source array.

“Spread control element” means a spread component that is controllableand is capable of causing a spread component to change coordinates,either cross-line or inline.

“Spread control element operating states” means measurements givinginformation relevant to a spread model (such as a hydrodynamic forcemodel). Examples include winged body orientation, water flow rates overdeflectors, wing angles relative to a wing housing body, rudder angle,propeller speed, propeller pitch, tow cable tensions, etc.

“Spread control element performance specifications” or “performancespecifications” means the performance limits of the spread controlelements, both the individual elements and the system resulting from thecombination of all the spread control elements. Examples include therange of wing angle values possible for a winged control element, thetension limits for a towing cable, the stall angle of a deflectordevice, etc.

“Spread front end” means the line (best fit or actual) formed byconnecting the front end of the streamers, more or less perpendicular tothe course made good of the vessel.

“Spread model” or “model of the spread” means code that is readable andexecutable by a computer for simulating the response of the spread tovarious input forces and conditions. A spread model may be ahydrodynamic force model, a neural network system, a closed loop controlsystem (see, e.g., International Patent Application No. WO 00/20895), amotion model driven and calibrated by an L-Norm best-fit criteria, or aKalman filter.

“Steerable front-end deflectors” (a.k.a. SFEDs) means steerabledeflectors positioned at the front end of the outer most streamers, suchas WesternGeco's MONOWING™ devices.

“Steered feather” is similar to natural feather, but with the angle isaltered by steering devices.

“Steering devices” means devices for steering at least one of the spreadcomponents. Such devices include streamer steering devices, steerablefront-end deflectors, and steerable buoys.

“Streamer steering devices” (a.k.a. SSDs) means steering devicesdistributed along the streamers, such as WesternGeco's Q-FIN™ devices.

“Tow points” are the points of origin on a towing vessel for the towedspread objects (e.g., points where lead-in cables exit the block on theback deck).

“Track” means the pre-designated two-dimensional coordinates for aspread component to occupy while conducting a portion of a seismicsurvey, such as a seismic survey line. Examples include a pre-plot lineor a non-straight pre-survey set of coordinates.

“Trajectory” means the realized or actual set of coordinates that anyspread component occupies during the survey.

“Translate” means an origin shift in x and y coordinates that gives anew origin for navigation purposes.

“Transform function” means a series of computations taking place in acomputer that has various measured or projected quantities as input, anda set of drive commands that are designed to give a computed and desiredchange in positions of any number of objects as output.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a method for controlling aseismic survey spread while conducting a seismic survey, the spreadhaving a vessel, a plurality of spread control elements, a plurality ofnavigation nodes, and a plurality of sources and receivers. The methodincludes the step of collecting input data, including navigation datafor the navigation nodes, operating states from sensors associated withthe spread control elements, environmental data for the survey, andsurvey design data. The positions of the sources and receivers areestimated using the navigation data, the operating states, and theenvironmental data. Optimum tracks for the sources and receivers aredetermined using the position estimates and a portion of the input datathat includes at least the survey design data. Drive commands arecalculated for at least two of the spread control elements using atleast the determined optimum tracks.

The estimating, determining, and calculating steps of the inventivemethod may be executed by a transform function. More particularly, thepositions may be estimated according to a spread model within thetransform function. In one embodiment, the spread model calculates afirst set of estimated positions using input that includes at least theoperating states and the environmental data. The collected navigationdata includes a second set of estimated positions. The first and secondsets of estimated positions are combined within the transform functionto produce the estimated source and receiver positions and predictedresiduals. The predicted residuals are used to estimate a set ofparameters that characterize the spread model. The spread modelparameters are used to calibrate the spread model. The predictedresiduals may further be used to estimate error states for sensors usedto collect the environmental data.

The optimum tracks may be determined according to a weighting functionwithin the transform function. In one embodiment, the weighting functionreceives as inputs the survey design data and the estimated positions ofthe sources and receivers. The input from the survey design data mayinclude performance specifications for the spread control elements, suchas steering constraints. In this embodiment, the weighting function isused to apply relative weighting coefficients to the inputs forcalculation of optimum tracks for the spread by the transform function.

In a particular embodiment of the inventive method, the spread model isa hydrodynamic force model of the spread components. The force model maybe based upon marine current data, among other things. In otherembodiments, the spread model is a pure stochastic model of the spreadcomponents, is a neural network, or employs one of the L-norm fittingcriteria. All these embodiments have in common the ability toparameterize control of the spread learned from a history of behaviorbased on all inputs, and an ability to generate drive commands that willrealize an optimum set of spatial targets, either in the form ofcoordinates (e.g., shot point targets) or shape (e.g., steered feather),for the spread in the future.

In a particular embodiment, the spread response times are estimated andtaken into account when calculating the drive commands. In thisembodiment, the drive commands are also regulated to maintain stabilityof the spread and validated before being delivered to the spread controlelements. Drive commands—particularly those used to control thevessel—can be implemented manually or automatically. Since most drivecommands will have a slow response time, the implementer will be a humanoperator in some instances. Other drive commands, such as SSD wing anglechanges, will preferably be controlled automatically, as is described inInternational Patent Application No. WO 00/20895.

The drive commands may be validated according to geophysical andoperational requirements. The geophysical requirements include achievingdesired coverage of a subsurface area, duplicating the seismic signalray paths of a prior survey, and controlling the seismic sensor noise.The operational requirements include defining one or more safe passagesfor the spread through dangerous areas, determining an optimum time toperform one or more lines of the survey, and reducing non-productivetime. Accordingly, alternative drive commands may be calculated foreffecting a safe passage between two or more definable locations.

Once validated, the drive commands are delivered to the spread controlelements for attaining desired survey objectives. The drive commands mayinclude commands for controlling at least one of the vessel propeller,vessel thruster, spread component steering devices, and the vessel cablewinches.

Each of the drive commands is preferably used to control at least one ofthe position, speed, and heading for one or more components of thespread. The spread components typically include one or more marinevessels, and a plurality of components towed by at least one of thevessels. The towed components typically include cables, sources, sensorssuch as hydrophones, and steering devices such as steerable front-enddeflectors (SFEDs) and streamer steering devices (SSDs). The spreadcomponents may further include one or more vehicles not tethered to theone or more vessels, such as an autonomous underwater vehicle (AUV) oran autonomous surface vehicle (ASV).

The spread control elements include at least two of a rudder, apropeller, a thruster, one or more devices for steering towed cables andinstruments, and one or more steerable flotation devices. The sensorsassociated with the spread control elements for producing operatingstates collected among the input data include one or more sensor typesof tension, water flow rate, inclination, orientation, acceleration,velocity, and position.

The environmental data collected among the input data include one ormore data types of current, salinity, temperature, pressure, speed ofsound, wave height, wave frequency, wind speed, and wind direction.

The survey design data collected among the input data includes one ormore data types of area, depth, area rotation or shooting orientation,line coordinates, source and receiver positions, required coverage,local constraints, optimizing factors, and historical data. The surveydesign data further includes performance specifications for the spreadcontrol elements, such as drag and maneuvering characteristics for thevessel, steerable cable devices, steerable source devices, anddeflectors, drag characteristics for the towed cables, sources, andflotation devices, and winch operating characteristics. The surveydesign data may also be characterized by the spread tracks, performancespecifications, and survey objectives.

The set of collected input data may also be characterized as includingone or more data types of pre-survey, operator input, present survey,near real-time or real-time survey, and simulated survey.

The pre-survey data may include environmental sensor data and historicalsurvey data.

The operator input data may include spread parameter settings andenvironmental data.

The real-time survey data may include one or more data types of cabletension, water flow rate, inclination, orientation, acceleration,velocity, positioning, spread control element setting, environmentaldata, seismic signal and noise data, and operator input. The collectedpositioning data may include data from one or more sensors of the groupconsisting of GPS receivers, echo sounders, depth sensors, acousticranging systems, magnetic compasses, gyro compasses, radio-locationsystems, accelerometers, and inertial systems. The spread controlelement setting data may include one or more inputs of the groupconsisting of thruster setting, propeller pitch, propeller rotationspeed, rudder angle, towing cable tension, winch position, deflectororientation, deflector angle of attack, deflector water speed, streamersteering device orientation, and streamer steering device wing angle ofattack.

The simulated survey data may include one or more data types ofsimulated pre-survey, simulated operator input, simulated presentsurvey, simulated near real-time, simulated real-time survey, andsimulated environmental data.

The raw seismic sensor data collected during the seismic survey may alsobe characterized as input data. Accordingly, in one embodiment, theinventive method further includes the step of using the raw seismicsensor data to produce quality indicators for the estimated positions.The quality indicators may include binning datasets, absolute noisedata, signal-to-noise ratios, and seismic signal frequency content. Thequality indicators may be used to validate the real-time survey data,spread control operating states, and drive commands.

In another aspect, the present invention provides a system forcontrolling a seismic survey spread while conducting a seismic survey,the spread having a vessel, a plurality of spread control elements, aplurality of navigation nodes, and a plurality of sources and receivers.The system includes a database for receiving input data includingnavigation data for the navigation nodes, operating states from sensorsassociated with the spread control elements, environmental data for thesurvey, and survey design data. The system further includes: acomputer-readable medium having computer-executable instructions forestimating the positions of the sources and receivers using thenavigation data, the operating states, and the environmental data; acomputer-readable medium having computer-executable instructions fordetermining optimum tracks for the sources and receivers using theestimated positions and a portion of the input data that includes atleast the survey design data; and a computer-readable medium havingcomputer-executable instructions for calculating drive commands for atleast two of the spread control elements using at least the determinedoptimum tracks.

In one embodiment of the inventive system, the position-estimatinginstructions, the optimum track-determining instructions, and the drivecommand-calculating instructions are contained in a commoncomputer-readable medium.

In a particular embodiment, the inventive system further includes acomputer-readable medium having computer-executable instructions forvalidating the calculated drive commands, and a network for deliveringthe validated drive commands to the spread control elements, whereby adesirable survey objective may be attained.

The inventive system otherwise contemplates and includes features of theinventive method summarized above.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the above recited features and advantages of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference to theembodiments thereof that are illustrated in the appended drawings. It isto be noted, however, that the appended drawings illustrate only typicalembodiments of this invention and are therefore not to be consideredlimiting of its scope, for the invention may admit to other equallyeffective embodiments.

FIG. 1A is a plan view of a seismic survey spread for conducting amarine seismic survey.

FIG. 1B is an elevational view of the spread shown in FIG. 1A.

FIG. 2 is a flow diagram of a method of for controlling the spread inaccordance with one aspect of the present invention.

FIG. 3 is a schematic representation of a towed streamer exhibiting aconstant feather.

FIG. 4 is a schematic representation of a plurality of streamersexhibiting a constant separation mode.

FIG. 5 is a schematic representation of optimum streamer shape modelingwith local feather angles defined by segments along the streamer toachieve a best fit for a prior streamer survey shape.

FIG. 6 is a schematic representation of a best-fit straight lineaccording to a look-ahead projection of four shot points, wherein theresidual projections are recomputed based on the location after eachshot point.

FIG. 7 is a schematic representation of a combination of successivelook-ahead best-fit straight lines like that of FIG. 6.

FIGS. 8A-8B are schematic representations illustrating how acurrent-induced source cross-line shift can be expressed in terms ofsource feather angle, and current and vessel-velocity vectorresolutions.

FIG. 9 is a schematic representation of a correction or change instreamer front end that results in the streamer front end being offsetat an angle to the course made good, in order to overcome acurrent-induced crab angle θ.

FIG. 10 schematically shows the streamer front end centers being fittedto a desired steering track.

FIGS. 11 and 12 schematically illustrate how “best fitting” lines forbase survey streamers that have common slopes can be estimated andconverted to a common feather angle for all streamers at each shot.

FIG. 13 schematically illustrates the principal of FIGS. 11-12 beingapplied to estimating an optimum slope for individual streamers.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-1B illustrate a typical marine seismic acquisition surveyspread (also known simply as “spread”) 10 for performing 3-D surveys.The spread 10 is characterized by a plurality of components, some ofwhich are controllable and known as spread control components. Thespread components will typically include one or more marine vessels 11,such as the vessels described in U.S. Pat. No. 6,216,627, and aplurality of components towed by at least one of the vessels. The towedcomponents include cables such as lead-in cables 20, spreader lines 26,streamers 18, and source tow cables and pressure lines (both representedas 15), as well as sources 16, hydrophone sensors 21 within thestreamers, and steering devices such as deflectors 22, streamer steeringbirds 38, and source steering devices 17.

The spread components may further include one or more vehicles (notshown) not tethered to the one or more vessels, such as the unmannedpowered vessel described in U.S. Pat. No. 6,028,817, the autonomousunderwater vehicle described in U.S. Pat. No. 6,474,254, or the seabedtractor described in International Application No. PCT/GB01/01930 (WO01/84184).

The spread control elements typically include at least two of a rudderR, a propeller P, a thruster (not shown), one or more devices 17, 22, 38for steering the towed cables and instruments, and one or more steerableflotation devices 46, 52.

More particularly, in the case of a Q™ vessel owned and operated by theassignee of the present invention, the vessel 11 is provided with a GPSreceiver 12 coupled to an integrated computer-based seismic navigation(TRINAV™), source controller (TRISOR™), and recording (TRIACQ™) system14 (collectively, TRILOGY™), and tows a plurality of seismic sources 16,typically a TRISOR™-controlled multiple air gun source of the kinddescribed in our U.S. Pat. No. 4,757,482, and an array 19 of foursubstantially identical streamers 18. However, it will be appreciatedthat, in practice, as many as twenty streamers can be towed, for exampleby using the techniques described in International Application No.PCT/IB98/01435 (WO 99/15913) assigned to the assignee of the presentinvention. The streamers 18 are towed by means of their respectivelead-ins 20 (i.e., the high strength steel or fiber-reinforced cableswhich convey electrical power, control, and data signals between thevessel 11 and the streamers 18). The span of the outer-most streamers 18is controlled by two steerable front-end deflectors (SFEDs) calledMONOWING™ deflectors, indicated at 22, connected to the respectiveforward ends 24 of the two or more outer-most streamers. The SFEDs 22,which are described in detail in U.S. Pat. No. 5,357,892 assigned to theassignee of the present invention, act in cooperation with respectivespreader lines 26 connected between the forward end 24 of eachouter-most streamer 18 and the forward end 24 of its adjacent streamerto assist in maintaining a substantially uniform spacing between thestreamers 18.

Each streamer 18 includes a plurality (up to 4000) hydrophone sensors 21distributed at spaced intervals along the streamer's length. Each of thehydrophones 21 is separately wired so that its output signal can beseparately digitized and filtered, thereby permitting sophisticatedprocessing known as digital group forming, as described in InternationalApplication No. PCT/GB99/01544 (WO 99/60421) assigned to the assignee ofthe present invention.

Each streamer 18 is made up of a large number of substantially identicalstreamer sections connected together end to end. Each streamer sectioncomprises an outer plastic skin that contains several elongate stressmembers, e.g., made of Kevlar, and the hydrophones 21 which areseparated by kerosene-saturated plastic foam spacer material, asdescribed in U.S. Pat. No. 6,477,111 assigned to the assignee of thepresent invention. Alternatively, each streamer section may employ a“solid” construction such as the commercial offerings of Sercel andThales Underwater Systems.

Each streamer 18 further has a plurality of inline streamer steeringdevices (SSDs) 38, also known as “birds,” preferably Q-FIN™ birds of thekind described in U.S. Patent Application No. US 20020126575,distributed at 200 meter intervals therealong for controlling thestreamer's depth and steering it laterally. Additionally, each streamer18 has inline acoustic emitters or “pingers” 40 uniformly distributedtherealong, the pingers being interleaved between the birds 38. Thepingers 40 are part of a positioning and navigation system that isdescribed further below.

The rearward ends 42 of the streamers 28, i.e., the ends remote from thevessel 11, are connected via respective stretch sections 44 similar tothe stretch sections 36 to respective tailbuoys 46. The tailbuoys areprovided with respective pingers 48, similar to the pingers 40, andrespective GPS receivers 50.

The array 16 is further provided in the region of its forward end 24with additional buoys or floats 52. More specifically, the furtherfloats 52 are respectively connected to the streamers, often the 4outermost, 18 at respective watertight electro-optical “tee” connectors54 positioned between the two stretch sections 36 at the forward ends 24of the outermost streamers, so as to be towed by the streamers. Thebuoys 52, which can be substantially identical to the tailbuoys 46, areprovided with respective pingers 56 and GPS receivers 58, and areconnected to their respective connectors 54 by respective stretchsections 60. Although the buoys 52 are shown in FIG. 1A as offset withrespect to their streamers for clarity, in practice they aresubstantially in line with the streamers 18.

The seismic sources 16 are also provided with a GPS receiver, indicatedat 62, and an acoustic receiver such as a hydrophone 21. The sources 16are steerable via steering devices 17, such as the devices described inU.K. Patent Application No. GB 0307018.2 assigned to the assignee of thepresent invention.

In use, the seismic sources 16 and the seismic streamer array 19 aredeployed from the vessel 11 and towed at about 5 knots substantially inthe configuration shown in FIGS. 1A and 1B. The seismic sources 16 areperiodically fired, e.g., every 10 seconds or so, and the resultingreflected seismic data signals are detected by the hydrophones 21 in thestreamers 18, then digitized and transmitted to the system 14 in thevessel 11 via the lead-ins 20.

Although the sources 16 and the streamers 18 are shown in FIG. 1A asextending in perfectly straight lines behind the vessel 11, in practicethey are frequently subject to lateral displacement, due for example towind and wave action and currents (as described further below). Thus, inorder to build up an accurate positional representation of the earthstrata in subsurface area being surveyed, it is essential to determineaccurately the respective absolute positions (i.e., in latitude andlongitude) of the sources 16 and the hydrophones 21 for each shotproduced by the sources. This has typically been done for the sources 16using the GPS receiver 62. The respective positions of the hydrophones21 are determined with respect to one or more of the GPS receivers 50,58 and 62 by triangulation, using an acoustic ranging and positioningsystem based on the pingers 40, 48 and 56 operating in conjunction withselected ones of the hydrophones 21, as described in U.S. Pat. Nos.4,992,990 and 5,668,775, both assigned to the assignee of the presentinvention. Thus a completed seismic survey results not only in a vastamount of seismic data, but also a vast amount of positional datadefining the respective positions of sources 16 and the hydrophones 21for each shot produced by the sources. From this positional data (a.k.a.navigation data), the shape of the path or track followed by eachstreamer 18 throughout the survey can be determined.

With reference now to FIG. 2, the present inventive method includes thestep 110 of collecting input data, including navigation data 112 for thenavigation nodes, operating states 116 from sensors associated with thespread control elements, environmental data 118 for the survey, andsurvey design data 120. The set of collected input data may be acquiredfrom pre-survey information, operator input, the present survey (nearreal-time or real-time), and from simulated survey information.

Navigation Data

Navigation data 112 is available from the spread 10, as described above,through the determination of the three vectors of position, velocity,and acceleration for a plurality of points (navigation nodes). Subsetsof the seismic hydrophones along the streamer are designated as acousticpositioning receivers. These receive a unique acoustic signal frominline transmitters typically every 400 meters along the streamer.Combined, the transmitters and receivers give acoustic reference pointstypically less than each 100 meters along any streamer, as described inU.S. Pat. No. 5,668,775. The end points of the streamers are controlledby GPS reference points that tie the acoustic navigation nodes to theEarth Centered Earth Fixed coordinate system. The connection between theGPS references and the acoustic nodes is made through a combination ofknown distances, acoustically measured distances, and directionsmeasured by compasses. The totality of these measures is used to givecoordinate estimates to each of the navigation nodes in a least squaresadjustment computed at each shot point aboard the vessel.

The density of these navigation nodes and precision of the positionestimates are sufficient to give an adequate picture of the overall andlocal spread components. These navigation data are measures of thepositional responses of the spread 10. The three navigation-basedvectors can also be used to calibrate local inertial navigation devices.These local devices can give precise estimates of position, velocity,and acceleration to the spread control system, allowing the system tocalibrate itself at a higher frequency than the acoustic networkposition updates are available. The navigation updates are also usefulto calibrate the inertial devices themselves, which typically sufferfrom an accumulating error, commonly called drift. Calibration isfurther discussed in greater detail below.

Operating States

The sensors associated with the spread control elements for producingthe operating states 116 collected among the input data include one ormore sensor types of tension, water flow rate, vertical inclination,body orientation, acceleration, velocity, and position. These sensors ormeasurement devices will, in one embodiment of the present invention(described below), provide input to a hydrodynamic spread model that isused to describe the dynamics of the spread 10.

One set of operating states pertains to the vessel 11. These include thevessel heading, speed, rudder angle, propeller pitch, and vessel motion(i.e., heave, pitch and roll). Changes in these will result incross-line and in line coordinate changes in the tow point locations atthe rear of the vessel 11.

Another set of operating states relates to the steering devices 17, 22,and 38, and describes the water velocity over a lifting body such as adeflector wing. The sensors give the orientation of the device 22, e.g.,relative to a course made good and water speed over the lifting bodies.The sensors further indicate the wing angles and changes in wing anglesin relation to the water flow.

These operating states can be translated into forces exerted by thesteering devices. The sum of these forces, distributed over the lengthof a streamer 18 or connected to the points on the source array, and inopposition to the water induced forces against the towed body surfacearea, (gun array floats for example also called sausages), give:

1. streamer shape starting from the tow point (origin);

2. center of source; and

3. individual source array positions relative to their vessel towpoints.

Tension on the towing cables is another important operating state thatis input—in one embodiment—to a hydrodynamic model. This ispredominantly a function of water velocity relative to the bodiesattached to the tension meters, and drag. In addition, tension is usedto determine if the towing lines are approaching their limits,constraining the amount of steering forces to be exerted by the steeringdevices.

Winch counters report the length of towing cable deployed, which, whencombined with the SFED forces, determines the orientation of the spreadfront end.

These, and other various operating states, may be combined in the forcemodel to give the force vectors that determine the shape of the spreadcomponents under tow. This is described further below in reference tospread models such as the force model.

Environmental Data

The environmental data 118 collected among the input data include one ormore data types of current, salinity, temperature, pressure, speed ofsound, wave height, wave frequency, wind speed, and wind direction. Thecollected data includes pre-survey and present survey data.

The tidal currents in the area can be predicted using pre-survey tidalcurrent tables published by several sources. These include the BritishAdmiralty, the National Oceanographic and Atmospheric Administration(NOAA), the Service Hydrographique et Oceanographique de la Marine(SHOM). For areas where there is thought to be a strong tidal currentregime, the survey lines will be scheduled to coincide periods of lowcurrent. Periods of high current will, to the extent possible, be usedfor other survey maneuvers such as turns and run in.

Further, a survey history of the area can be reviewed to identify thehistoric degree of feather experienced in the survey area. Featherstatistics may be archived in a database for subsequent use. Feather isan indirect measure of current in a survey area. This measure can beused to indicate magnitude, direction and temporal and spatial rates ofchange in the area. Spatial frequency is related to streamer length.Feather can give an indication of spatial frequency by relating thespeed made good of the streamer tail to rate of change in feather. Ratesof change in feather will give the survey planners an idea of theresponse time required for the spread control system they arespecifying.

Units of time in the seismic data acquisition process are typically shotpoints. Long period can then be defined as some number of shot pointsinto the future corresponding to the length of time the presentenvironmental conditions will persist.

As an example, in tidal shooting the tidal current cycle times are wellknown. Seismic lines have for at least 15 years been planned to get thesame current or temporal current gradient along adjacent lines in orderto reduce infill. Several seismic exploration software providers offersurvey line planning software to anticipate temporal and spatial currentchanges during seismic acquisition.

In addition, any historical current data available may be reviewed toidentify the direction of the strongest currents. If the geophysicalobjectives allow, the line directions are preferably planned to beparallel to the predominant current direction. This will give the leastfeather and straightest streamers. Such data is available in mature oilproducing areas due to the need for current knowledge for rig andfloating production storage offloading (FPSO) maneuvers.

Several measurement sources of current data are available formeasurement during a survey. Vessel hull mounted Acoustic DopplerCurrent Profilers (ADCPs) measure current some hundreds of meters beforethe source array and spread front end. Current meters mounted onsemi-permanently or fixed structures in the survey area, (e.g., bottommounted rigs and FPSOs) can report local current via a telemetry link tothe vessel 11 in real time. Work or chase boats, or any other mobileplatform including remotely operable vehicles (ROVs), havingcurrent-measuring devices aboard can precede the spread 10 along thesurvey track and telemeter the current regime the spread will encounterin the future. Satellite imagery provides knowledge of macro scale loopcurrents and warm water mass eddies.

All sources of current are stored in a Geographic Information System(GIS) database with a time tag. This type of system is commonly used tomanage spatially distributed data. An example is the type of datamanagement system used by Horizon Marine. For short periods the data canbe considered valid, (e.g., an hour or less). Longer period trends canbe derived based on the historical changes observed throughout thecourse of data acquisition and used to anticipate conditions on adjacentlines. Further, the tidal driven component of current that was predictedas described above can be calibrated based on in situ measurements. Thefrequency content of the tidal signal being known, the amplitude andphase shifts predicted from tables can be adjusted to fit the exactlocale of the survey.

In situ wind meter data, obtained from meters or sensors located on thesame platforms mentioned above for current, can be treated exactly ascurrent meter data. The use of this data is of course to model forcesexpected on objects on the sea surface. In addition, surface layer watercan be moved by air friction and cause surface wind driven current. Theeffects of wind-driven surface currents reach down to several meters,which is presently the zone for towing streamers.

Dynamic oceanographic models of ocean cubes, such as those offered byHorizon Marine, can be used to predict various ocean phenomena. Thesemodels are roughly equivalent to weather prediction models and areanalogous in their accuracy of prediction as a function of time. Thesemodels require inputs such as current measures, and wind for theircalibration and boundary conditions. Two of the main drivers for thesemodels are water density differences and earth motion (i.e., Coriolisforce). Density differences are inferred from temperature, pressure(depth), and salinity data collected horizontally throughout the surveyarea and vertically through the water column by probes that are eitherdisposable or retrievable. These data map the density interfaces thattogether with earth rotation, wind and other forces cause water bodiesof different densities to move in relation to one another. The verticaldensity gradient is largest in the upper layers due to solar warming andnear land where water originating from land enter the sea and wherevertical land masses cause water of differing densities to change depth,(e.g., coastal upwelling).

Dynamic oceanographic models are well known but are often macro scale(i.e., areas many times larger than a survey area). Recent advances incomputing power have lead to the development of models suitable formeaningful prediction of water body movement in areas on the scale of asurvey area. Typical numerical models are described in IntroductoryDynamical Oceanography by Pickard and Pond. The use of modeling topredict currents that will be encountered by a spread acquiring aseismic survey may be applied in situ to anticipate current. In situcurrent and wind measurements will also be used to calibrate theoceanographic model predictions. Greater frequency and horizontalexpanse of the density measurements results in better resolution of thewater mass boundaries and improved modeling and calibration.

Any subset of current determination methodologies described above, withany degree of calibrated modeling, or un-calibrated modeling, as well asdirect measurement, is valuable for acquisition since it can reduceacquisition time by increasing production time. The older the data, theless valuable it is. Presently-obtained information (near-real timeand/or real time) will be used to estimate forces that will beencountered along the acquisition line.

The collection of water density data as described above is presently andwill continue to be used to estimate acoustic wave front propagationbetween source and receiver points defined as navigation nodesthroughout the spread.

Wave height measurement can be obtained from satellite imagery as wellas in situ from heave meters and high-frequency GPS vertical velocityestimates. Changes in water column position have an impact on seismicrecording and this fact is responsible for the depth-keeping requirementimposed on the SSDs. Wing angle changes for the purpose of controllingdepth will impact a steering device's ability to steer laterally.Currently, the Q-FIN™ SSD controller combines horizontal and verticalpositioning. Knowledge of wave height aids in determining the availablelateral steering ability while maneuvering the streamer. Wave heightgives a measure of water particle motion in three dimensions through thewater column. This is in effect a small scale current. The amplitude ofthe wave height will dictate whether current is a significant force atstreamer depth.

Input Data Collection

The pre-survey data collected among the input data preferably includesenvironmental sensor data. The portion of the input data 110 that iscollected as real-time survey data may include one or more data types ofcable tension, water flow rate, inclination, orientation, acceleration,velocity, positioning, spread control element setting, environmentaldata, seismic signal and noise data, and operator input. The collectedpositioning data may include data from one or more sensors of the groupconsisting of GPS receivers, echo sounders, depth sensors, acousticranging systems, magnetic compasses, gyrocompasses, radio-locationsystems, accelerometers, and inertial systems. The spread controlelement setting data may include one or more inputs of the groupconsisting of thruster setting, propeller pitch, propeller rotationspeed, rudder angle, towing cable tension, winch position, deflectororientation, deflector angle, deflector water speed, streamer steeringdevice orientation, and streamer steering device wing angle. Theoperator input data may include spread parameter settings andenvironmental data.

The simulated survey data may include one or more data types ofsimulated pre-survey, simulated operator input, simulated presentsurvey, simulated near real-time, simulated real-time survey, andsimulated environmental data.

The raw seismic sensor data collected during the seismic survey may alsobe characterized as input data. Accordingly, in one embodiment, theinventive method further includes the step of using the raw seismicsensor data to produce quality indicators for the estimated local waterflow at the streamer surface. The raw seismic sensor data is useful toverify the force model and current expectations. Measured ambient noiseis compared to predicted or expected ambient noise given the expectedwater flow over the surface of the streamer. Large differences betweenthe expected and recorded noise indicates either the recording system isin error or the current is different than anticipated. Changes inambient noise along the length of the streamer gives the spatial currentgradient The quality indicators may include binning datasets, absolutenoise data, signal-to-noise ratios, and seismic signal frequencycontent. The quality indicators may be used to validate the real-timesurvey data.

Survey Design

The survey design data 120 collected among the input data includes oneor more data types of area, depth, shooting orientation, linecoordinates, source and receiver positions, required coverage,constraints, optimizing factors, and historical data. Those skilled inthe art will appreciate that the survey design data further includesspread performance specifications 114, as described below. The surveydesign data may also be characterized by survey objectives andconstraints, and may be substantially defined by pre-survey information.

Survey design is relevant since geophysical objectives are constraintswithin which all seismic surveyors must work. General survey design willencompass all aspects of a survey objective. Certain of the geophysicalobjectives will impact the acquisition. These include:

1. number and length of streamers;

2. streamer separation;

3. source array dimensions;

4. shot point spacing; and

5. line direction

Once the geophysical objective(s) for the survey design have beendetermined, it becomes important to identify the factors that will makethe seismic data acquisition difficult and try to mitigate them. If, forexample, an objective of the survey is time lapse (4-D), a factor makingacquisition difficult is a non-straight prior or base survey trajectory.Knowledge of the trajectory is gained by reading the “P190” dataproduced from the prior survey. These trajectories may then be comparedto a trajectory that can likely be acquired considering the selectedacquisition hardware. If, however, a principal objective is conventionalcoverage, pre-plot lines will determine the survey tracks. For anygeophysical objective, however, local obstructions and bathymetry willbe constraints to the planned tracks.

The above description of the use of survey design data, spread controlelement specifications, environmental data, and the operating statesparticularly apply (but are not limited) to measurements taken duringthe present survey. These data are input to a general transform function121 that gives a set of desired output, as shown in FIG. 2 and describedfurther below.

The spread control elements selected for the survey design will bechosen to meet the anticipated seismic data acquisition requirements. Inaddition, the vessel track will be constrained by the survey objectives.Further, obstructions in the survey area, and bathymetric data will bemonitored for proximity to the spread during the survey operation.

Performance Specifications

The performance specifications 114 collected among the survey designdata 120 is typically hydrodynamic, and may include vessel profile andcharacteristics, vessel maneuvering limitations, towed cable drag andother physical characteristics, steerable cable device characteristics,source drag and other physical characteristics, steerable source devicecharacteristics, deflector characteristics, flotation device drag andother physical characteristics, and winch operation characteristics.Such individual device specification data is typically available fromthe manufacturer and/or from historical data. These inputs may, amongother uses, be combined with the geophysical survey objectiveconstraints to conduct a simulated survey that is useful for surveydesign and can give a provisional combined towing system behaviorspecification. Thus, for example, various spread requirements andspecifications may be defined before the survey takes place, such asspatial frequency of streamer steering devices along the streamer,number of steerable front-end deflectors to deploy, number of sourcesteering deflectors to deploy, and computing power required for expectedcycles times (related to current gradient). Further, simulations of thissort can be used to design spread components for improved controlperformance. Examples of parameters that could be varied in simulationsare, cable diameter, cable density, more hydrodynamic cable body shapesand steering devices.

Position Estimation

Having collected the input data, the positions of the sources andreceivers are estimated using the navigation data 112, the operatingstates 116, and the environmental data 118. More particularly, thepositions are estimated according to a spread model 123 within thetransform function 121. The spread model calculates a first set ofestimated positions using input that includes at least the operatingstates 116 and the environmental data 118. The environmental data isused as described in FIG. 9 to give the natural feather. Added to thenatural feather is some amount of steered feather, demanded from theSSDs 38. An example of an operating state contributing to positionestimation is a steering input/correction for achieving a desiredfeather angle. Steered feather is obtained by the exertion of force inthe cross-line direction by the SSDs along the streamer 18. The equationgoverning the exerted force is based on the wing lift equation:

$\begin{matrix}{L = {C_{l}*A*\rho*\frac{V^{2}}{2}}} & {{Eqn}\mspace{14mu} 1}\end{matrix}$

where:

C_(l)=lift coefficient;

A=wing surface area;

V=water velocity with respect to the wing angle of attack; and

ρ=water density.

The angle of attack is adjustable, and is thus another operating state.Changes in the angle of attack create an acceleration or change in forceexerted by the SSDs integrated or coupled to the streamers.

The collected navigation data 112 includes a second set of estimatedpositions. A subset of the seismic hydrophones 21 along the streamer aredesignated as acoustic positioning receivers. These receive a uniqueacoustic signal from inline transmitters typically every 400 metersalong the streamer. Combined, the transmitters and receivers giveacoustic reference points (i.e., navigation nodes) typically less thaneach 100 meters along any streamer, as described in U.S. Pat. No.5,668,775. The end points of the streamers are controlled by GPSreference points that tie the acoustic navigation nodes to the EarthCentered Earth Fixed coordinate system. The connection between the GPSreferences and the acoustic nodes is made through a combination of knowndistances, acoustically measured distances and directions measured bycompasses. The totality of these measures are used to give coordinateestimates (the second set of position estimates) to each of thenavigation nodes in a least squares adjustment computed at each shotpoint aboard the vessel.

The first and second sets of estimated positions are combined (see node122) within the transform function to produce the (combined) estimatedsource and receiver positions and predicted residuals (see box 122 a).The predicted residuals represent the difference between the first andsecond sets of estimated positions, and are used to estimate a set ofparameters that characterize the spread model 123. The spread modelparameters are used to calibrate the spread model. The predictedresiduals may further be used to estimate error states for sensors usedto collect the environmental data.

Optimum Track Determination

The optimum tracks are determined at 124 according to a weightingfunction 125 within the transform function 121. The weighting functionreceives as inputs the survey design data 120 and the most recentlyestimated positions of the sources and receivers (see box 122 a). Theinput from the survey design data may include performance specificationsfor the spread control elements, such as steering constraints. Othersurvey design criteria include geophysical and operational requirements.The geophysical requirements may, e.g., include achieving desiredcoverage of a subsurface area, or duplicating the seismic signal raypaths of a prior survey, and controlling the seismic sensor noise. Theoperational requirements may, e.g., include defining one or more safepassages for the spread through dangerous areas, determining an optimumtime to perform one or more lines of the survey, and reducingnon-production time. The weighting function 125 is used to applyrelative weighting coefficients to the inputs for calculation of optimumtracks for the spread by the transform function. “Optimum track”determination includes an optimum spread body shape determination, andthe corresponding shape change along a track.

In order to achieve the objectives of a seismic survey, some set ofcoordinates (i.e., a “track”) must be occupied. The first estimate of adesirable or “optimum” survey track is made in the survey design phasedescribed above. In situ, this track will be re-computed at somefrequency, depending on the forces present and the frequency ofnavigation updates. Even if the re-computation of the optimum trackoccurs at a high frequency, the response time of the system will beconsidered when issuing drive commands to optimally realize the optimumtracks. In areas of small current, the survey design track or pre-surveytrack may be achievable with little or no effort on the part of thespread control system. In other areas, a high frequency re-computationof the best-cost track may be required. The re-computation can only beachieved if there is a navigation update to reveal the success of thespread model-driven prediction. The re-computation is only needed if thenavigation update shows that the predicted trajectory has deviated fromthe track by more than the probable error bounds set (also referred toas the no-change corridor).

In practice, the physical constraints imposed by nature combined withthe steering system limits will likely prevent the intended pre-surveytrack from being followed to some degree. The path determination is madewith consideration for the target coordinates and the ability to reachthe coordinates given a potential for spread control.

In one embodiment of the optimum track computation 124, a best-cost mapmethod as described by U.S. Pat. No. 6,629,037, assigned to the assigneeof the present invention, is employed. The successive candidate cells(track segments) are weighted by a function that incorporates acombination of factors that may generally be characterized as steeringconstraints. These factors include:

-   -   1. pre-survey track of all spread components;    -   2. a separation of importance for spread components, analogous        to the offset weighting in Nyland;    -   3. the steering potential available;    -   4. the response time of the system;    -   5. the stability of the system; and    -   6. the physical limits of the system.

The optimum track is first checked for collision potential, with bothspread elements and external obstructions, before being forwarded to thespread model to be transformed into the drive commands that will realizethe optimum track. The track optimization safety criteria includeverification (see box 127) that the trajectory of any spread elementposes no danger of collision. A “no” result will cause feedback throughthe GUI to the user that either the steering constraint parameters arenot set correctly or that the optimization algorithm is flawed. The userthen has the option to take manual control of the steering system ormodify the steering constraints. Steering constraint modification is forexample, if the streamer separation limits are exceeded, the user mayopt to allow the streamers to pass closer to each other. For anotherexample, if a spread element (e.g., tailbuoy) will pass too close to anobstruction such as a Floating Production SO, the user may opt to havethe FPSO change position and enter this into the survey design data flowso that the optimum track may be realized safely.

A “yes” result to the safety check will lead to the submission of thedetermined optimum tracks to the spread model 123 for use in computingnew operating states (i.e., drive commands) for the spread controlelements.

The drive command optimization computation results in a set of drivecommands—primarily steering commands—that will bring about changes inthe positions of the spread components as part of the transform function121. The drive command optimizations will be constrained by theprojected environmental conditions and the steering devices available toenable the steering. The definition of optimization will be determinedby the optimum track.

Drive Command Calculations

Drive commands (also referred to herein as new operating states thatresult from the optimum track determination) are calculated in thespread model 123 for at least two of the spread control elements usingthe determined optimum tracks (from box 124) that have been validated(at 127). The spread response times are estimated by the spread modeland taken into account when calculating the drive commands. The drivecommands are also regulated to maintain stability of the spread, andvalidated (at 128) before being delivered to the spread controlelements.

Each of the drive commands calculated with the inventive method may beused to control at least one of position, speed, and heading for one ormore components of the spread. Typically, the drive commands willinclude commands for controlling at least one of the vessel propeller,vessel thruster, spread component steering devices, and the vessel cablewinches. The vessel cable winches, in particular, may be dynamicallycontrolled.

The optimization computation results in a set of drivecommands—primarily steering commands—that will bring about changes inthe positions of the spread components as part of the transform function121. The drive command optimizations will be constrained by theprojected environmental conditions and the steering devices available toenable the steering. The definition of optimization will be determinedby the objective(s) of the drive commands.

The optimization criteria include verification (see box 127) that anyset of mechanically-induced drive commands or force changes that arerequired to achieve a determined optimum track are within safetyrequirements for the survey. Typically, the safety requirements willfall into one of equipment safety constraints and human safetyconstraints. A “yes” result to the safety check will lead to thesubmission of the determined optimum tracks to the spread model 123 foruse in computing new operating states (i.e., drive commands) for thespread control elements. Thus, e.g., upon detection that certain of thespread control components have failed (such as the vessel propeller orrudder, deflectors, source or streamer steering devices), the systemwill assume a “maximum safety” mode that restricts drive commands in theinterest of equipment and personnel preservation.

Potential Determination

The potential for spread control is measured by the spread model 123,which in a presently preferred embodiment is a hydrodynamic force modelthat determines the amount of force available after subtracting theforce already consumed at the present shot cycle from the totalpotential force. Steering potential, while derived from available force,can be expressed in units of feather angle (e.g., degrees, or anyangular measure). Depending on the survey design, including theacquisition objective(s), an analysis is made to determine whether drivecommand changes are needed and, if so, which changes are appropriate.Force by definition has an acceleration component. The systemperformance capacity, including the steering potential available, ispredicted by the theoretical force-driven model and the spread controlelement drive commands that should give the necessary accelerations.

Delay, System Response, and Position History Relations and Error States

As mentioned previously, the position histories (first estimatedposition sets) predicted by the spread model 123 are compared withposition history estimates resulting from the navigation solution(second estimated position sets), forming the predicted residuals. Thepredicted residuals are then related to the error states defined withinthe force model inputs, the force model parameters, and the spreadcontrol element performance specifications. In an error-free model,predicted responses will occur on schedule, or, in other words, systemdelays will be accounted for in the predicted response. Before the modellearns from the navigation solution what the system responses are,through calibration, model predictions will have some degree of error,with the error magnitudes depending on the quality of the model andinputs.

Before a history of comparisons is available, the navigationsolution-based histories (second estimated position sets) will beinfinitely higher in weight compared to the force model-based positionhistories. Practically, this means the combined navigation and predictedmodel position estimates are equal to the navigation estimate withnearly all the predicted residual being attributed to the spread model.After the model is calibrated, the force model expectation of positionhistory should consistently agree with the navigation-based measuredhistory to within the expectation of error in the measured, ornavigation solution, position estimates.

Drive Command Calculations

Drive commands (also referred to herein as new operating states thatresult from the optimum track determination) are calculated in thespread model 123 for at least two of the spread control elements usingthe determined optimum tracks (from box 124) that have been validated(at 127). The spread response times are estimated by the spread modeland taken into account when calculating the drive commands. The drivecommands are also regulated to maintain stability of the spread, andvalidated (at 128) before being delivered to the spread controlelements.

Each of the drive commands calculated with the inventive method may beused to control at least one of position, speed, and heading for one ormore components of the spread. Typically, the drive commands willinclude commands for controlling at least one of the vessel propeller,vessel thruster, spread component steering devices, and the vessel cablewinches. The vessel cable winches, in particular, may be dynamicallycontrolled.

The drive commands are typically determined according to geophysical andoperational requirements. The geophysical requirements may, e.g.,include achieving desired coverage of a subsurface area, or duplicatingthe seismic signal ray paths of a prior survey, and controlling theseismic sensor noise. The operational requirements may, e.g., includedefining one or more safe passages for the spread through dangerousareas, determining an optimum time to perform one or more lines of thesurvey, and reducing non-production time. Accordingly, alternative drivecommands may be calculated for effecting a safe passage between two ormore definable locations.

Invention Applications Other than Real Time Surveying

An additional role of the present invention is to provide the operatorwith “intelligent finishing” or scenario-planning. The operatorexpresses basic intentions to the transform function 121 for a routebetween two or more points and the module evaluates possible safealternative passages for the entire spread which fall within the spreadsteering capabilities and presents them to the operator for selection.This could be used when arrival at a particular point at a particulartime is required. Another use could be when a safe close passage to apermanent or semi-permanent structure or feature in the survey area isrequired for operational reasons.

Intelligent-finishing uses the same extrapolation into the future butthe limits imposed on solutions are different to those used in asurveying environment. In this case the emphasis is on safety and traveltime rather than ensuring that each individual element of the spreadfollows an exact pre-defined path. It might be that exclusion zonesdefine areas that individual elements should not enter. Theextrapolation time will normally be longer and the uncertainties withinthe system which can be accepted are greater. In this case the operatorchooses which scenario to accept.

Still another application of the invention applies to developmentsimulation. Actual input data is run through the transform function 121with steering devices under development. Projected improvements inperformance are used to gauge the value of developing the steeringdevice improvements.

Based on the objective of the steering system, a vessel track, streamerfront end track, source track, and streamer feather may be computed togive the best positioning of the spread, driven by the spread controlelements. This will be described in greater detail below using a forcemodel as an exemplary spread model 123.

Applications Overview

The table that follows presents typical examples of optimizationcriteria according to broadly defined survey periods:

TABLE 1 Optimization Criteria Event Pre-Conditions Objective OutputConstraint Pre-Survey None Evaluate survey Survey shooting Avoid hazardto with required bin plan own or other coverage in equipment minimumnumber of vessel passes Pre-Survey None Determine Worst-case errorMaintain feasibility of ellipses; major stability of performing riskfactors, system successful survey spread control in a given areaelements or system required, maximum heading changes likely SurveyCurrent state of Complete survey Survey shooting Avoid hazard to surveywith required bin plan own or other coverage in equipment minimum numberof vessel passes Survey All gear in water; Move from current Steeringplan for Avoid hazard to ready to shoot position to start of vessel andown or other next pass equipment equipment Survey All gear in water;Move from current Steering plan for Avoid hazard to ready to shootposition to a vessel and own or other desired position equipmentequipment Survey All gear in water; Control vessel and Drive commandsAbide by pre- ready to other spread delivered to defined safetyshoot/shooting control components spread control margins according tocomponents original shooting plan Deployment/ All/some gear on Deploy astreamer Steering plan for Minimize risks recovery vessel vessel andequipment Deployment/ All/some gear in Recover a streamer Steering planfor Minimize risks recovery water vessel and equipment Deployment/ Allgear in water Streamer Steering Minimize risks recovery maintenanceinstruction to workboat; streamer commands to aid maintenance Change(s)Current state of Complete survey Survey shooting Avoid hazard to inspec, survey; changes with required bin plan own or other conditions,coverage in equipment environment minimum number of vessel passesFailure Failure of vessel Restore propulsion All control Maximizepropulsion systems set to safety for safest position equipment andpersonnel Failure Failure of Repair/replace All control Maximizedeflector failed deflector systems set to safety for safest positionequipment and personnel Failure Failure of vessel Restore steering Allcontrol Maximize steering systems set to safety for safest positionequipment and personnel Failure Failure of source Repair/replace or Allcontrol Maximize steering device make fail-safe the systems set tosafety for failed device safest position equipment and personnel FailureFailure of Repair/replace or All control Maximize streamer steering makefail-safe the systems set to safety for device failed device safestposition equipment and personnel

Accordingly, alternative drive commands may be calculated for effectingvarious spread trajectories.

Transform Function

As mentioned previously, the transform function 121 executes theposition-estimation, optimum track-determining, and drivecommand-calculation steps 122, 124, 126 of the inventive method. Thespread model 123, based on the inputs also mentioned earlier, generatesa first set of predicted position estimates and/or spread body shapesahead until the next navigation update. This set of predictions iscombined with the navigation system position estimates (a second set) toget the combined source and receiver position and/or shape estimates.The predicted residuals (difference between first and second sets) areused to estimate certain key spread model parameters, and any errorstates associated with the environmental measurements such as current orwind. The combined position estimates are delivered to the optimum trackestimation algorithm 124 and a weighting function 125.

The resulting spread model parameters are fed back to the spread modelalgorithm 123. Further, estimates of the environmental measurement errorstates are fed back as calibration values to the environmental measuringdevices (see box 118).

The optimum tracks are preferably determined at 124 according to aweighting function 125 within the transform function. In a particularembodiment, the weighting function receives as inputs the survey designdata 120 (including the performance specifications), as well as thecombined position estimates. The weighting function assigns relativeimportance, or weights, to each of the combined position estimates andthe survey design data 120 (including, in particular, the steeringconstraints) to derive an optimum track or shape for the spread.“Optimum” in this sense means satisfying as closely as possible both thesteering constraints and survey objectives, given the present spreadposition estimates.

Besides the previously mentioned force model, the spread model may bedriven by a pure stochastic model of the spread components, it may be aclosed loop control system as described in International PatentApplication No. WO 00/20895 (PID controller based on a force model), aneural network, or it may employ one of the L-norm fitting criteria toestimate spread behavior. Essentially, any estimation theory methodsuitable for optimized coordination of a suite of spread controlelements may be applied to achieve a desired track for all or part ofthe spread. For the case of a neural network, the spread model ispatterned on the teachings of U.S. Pat. No. 6,418,378 (training modelusing “snapshot” spread coordinates).

If the transform function determines (box 124) that a substantiallydifferent spread shape or track is required or desirable, this spreadalteration is checked or validated by an internal safety check (see box127).

If the safety check determines the new track or shape is safe (“yes”),the coordinate set or shape description for the spread control elementsthat comprise the newly estimated optimum track are fed to the spreadmodel 123 to obtain the appropriate drive command corrections. As anexample of an optimized next step, the determination of whichcontrollable device or devices should be commanded is undertaken. Aninitial search is made using the principle that the device to becommanded is the lowest in the chain that can affect all out-of-limit orundesirable conditions. Thus, if the streamer array 19 and the sourcearray 16 are out of position in the same direction, changing theposition of the vessel 11, the “parent” towing device, is most likely tobe the optimum strategy. If the streamer and source arrays are out ofposition in opposite directions, changing individual controls on eachsubsystem might be optimum. An optimum change is computed, using therelationships established earlier, for example, a one degree rudderchange might change the vessel lateral motion by 0.1 meters on averageover five seconds.

For validation, this change is then extrapolated forward in time tocheck the effects on the entire spread over a period of time at leastcorresponding to next update cycle. If the effects are undesirable,another combination of control changes is established and theextrapolation process repeated. No matter what the limitations on thesteering available to the spread control elements, there is a definable,optimal steering command set. It may be that no change is possible thatachieves an initial definition of optimum or desirable results over theforward extrapolation period. In this case, the definition of optimum ismodified such that changes that achieve the desired results over thelongest period of time are searched for.

If the internal safety check 127 determines that the computed optimumtrack is not safe, this “no” response is fed back to the operator onlinethrough a Graphical User Interface (GUI). The operator is therebynotified of the track components that exceed the safety check limits andis prompted to modify the survey design to mitigate the safetyviolation. This may entail re-weighting certain target points along basesurvey. The operator has the option to take control of the system andsteer manually for the period of faulty steering to ensure no accidentsoccur.

In general, when a correction is needed, there are always two types ofcorrections that may be made: one which removes the source error bymaking a control change at or ahead (upstream) of a problem area; andone which removes the propagation of an error or problem by makingcontrol changes behind (downstream) the problem area.

When the drive commands are chosen by the spread model 123 to realizethe safe validated optimum track, the drive commands themselves arevalidated at 128. This is a failsafe mechanism for the drive commandchoice algorithm. If the drive commands are validated (“yes”), they aredelivered to the spread control elements to be carried out (see 130).

If the drive commands fail the validation step (“no”), the operator isagain alerted to an algorithmic failure and given the option to takemanual control or modify one of the parameters that make up the steeringconstraints.

The transform function cycle can occur as often as navigation data isavailable and computing power permits. Alternatively, the cycle can becarried out less frequently, and the spread model constantly re-issuesdrive commands that will cause the spread to conform to the mostrecently determined optimal shape or track.

Over time, an optimum spread model is developed through calibration,essentially a learning process, comparing measured position history withexpected model outcomes. This model will vary according to the equipmentwithin the spread and the prevailing sea and weather conditions. Thespread model 123 is primed with a coordinate set for the spreadcomponents to determine the starting-point in the model. It then buildsup a dynamic model view of the spread components based on prevailing seaconditions of currents and tides and the effects of spread controlelements, among other things mentioned herein, and calibrates orotherwise trains the spread model. System calibration is achieved byestablishing the relationship between the system parameters and thepredicted residuals. The cause of the predicted spread componentcoordinate changes will always be due to at least one of spread controlelement operating state changes, sea current and near surface wind. Thenatural forces can be known by direct measurement with current and windmeters, inferred by spread element changes measured with the navigationsolution, or by model prediction in an ocean and/or weather predictionmodel, such as the models available through Horizon Marine.

Long Term Planning

In areas where the most significant influences are generated by highlydeterministic and predictable phenomena such as tidal currents, theseinputs can be used to generate an optimal set of steering commands up tomonths before the actual survey time. The spread model 123 may thenextrapolate forward using the calibrated spread model parameters.

Real Time Adjustments to the Planned Optimal Track

During the execution of the survey, the optimum track, based on tidal orother currents, can be adjusted based on the actual positions realizedalong the survey line. A no-change scenario occurs when the actualtrajectory realized is within the limits set for the planned track. Theno-change corridor limits will be derived from both the error estimatesassociated with the combined source and receiver position estimates andthe steering constraints thought to give the optimum result. This checkcan occur for each cycle of the transform function.

If the trajectory falls outside the no-change corridor, then acorrection is required and the process shifts to the next stage. Thisnext stage is to reoccupy the no change corridor or some other corridor,perhaps narrower, but centered within the no-change corridor. An exampleof a methodology for keeping within the no-change corridor is throughPID control of the spread control elements. If the no-change scenario isachieved, then analysis is made of the way the scenario is changingtowards the limits to discover whether a correction would then bedesirable. The ideal solution is a scenario that remains mid-way betweenthe acceptable tolerance limits.

Transform Function Summary

Simply stated, the primary role of the transform function 121 is to takeall the available input data 110 and transform them into the necessarydrive commands for all the spread control elements to achieve a selectedsurvey objective. While there may be several possible solutions toachieve a near-instantaneous conformance to the requirements, thetransform function will calculate the solution set as it is projectedinto the future to ensure that steering commands made now will not causeunwanted effects for a time in the future. This time may, for example,be the time duration for the entire spread to pass over a givenlocation. During each cycle, the inputs are put to the transformfunction 121 which re-evaluates the current operating states and anyneed for a new, optimal set of operating states, including but notlimited to steering commands, and computes adjustments as necessary.

Calibration

As mentioned above, the spread model 123 is preferably calibrated usingspread model parameter estimates based on the predicted residuals and/ormeasured behavior, which are based on the navigation data 112.Calibration thus takes advantage of available measured outcomes, such aspositions obtained or estimated using another method such as acousticnetworks or GPS, to train the spread model before and after anyindividual or sequence of uncalibrated transform function cycle(s).

The calibrating step preferably includes minimizing the differencebetween the predicted residuals, by estimating spread model parametersthat result in agreement with the position estimate. In this manner,it's possible to feedback positioning quality information so that theaccuracy of the spread control components that contribute to theposition prediction process may be improved upon. Spread control modelparameters that might be calibrated include, towed body dragcoefficients, lift coefficients, current meter error, wind meter errors,operating state biases. Calibrating these parameters will narrow thedifference between track (pre-designated) and trajectory (actual)coordinates.

The minimization can be achieved by relating the hydrodynamic or othermodel type parameters mathematically to the observations that drive themodel. Current force in the sea and wind force on the sea surface arethe ambient or natural force regime while mechanical counter forcesgenerated by the spread control elements are used to position the spreadoptimally.

An example of spread behavior calibration through measured performanceis achieved steered feather. Given the range of demanded side forcesfrom the SSDs 38, a range of steered feather angles can be measured asan outcome. This outcome will be unique to the local current regime andthe spread under tow. Various streamer-steered feather angles can bepredicted and achieved as they were in the recent past, and thus the SSDresponse is calibrated. Similarly, the time taken to achieve varioussteered feathers can be measured and used to predict the feather changerequired to achieve optimum streamer target shapes.

The range of demanded side forces possible from any of the spreadcontrol elements is limited, especially in the framework of normal dataacquisition, (nearly straight tow). For this reason, only a small subsetof the total function that describes the temporal and spatial responseof the total system is needed for prediction of the small incrementalchanges demanded under normal operation.

Alternatively, the mathematical model fitting may employ a purestochastic model of the spread components. Other examples of themathematical model fitting steps may include one of the L-norm fittingcriteria, PID controller, Kalman filter, or a neural network, or anycombination of these.

System

In another aspect, the present invention provides a system forcontrolling the seismic survey spread 10. The system is preferablylocated onboard the vessel 11, but those skilled in the art willappreciate that one or more components may be located elsewhere such asanother vessel or on shore, as in remote monitoring of a survey from ashore based office, that includes some or all of the transform functioncomputations, depending on the data transfer rates available. The systemincludes a database for receiving the input data 110, and a set ofcomputer-readable medium(s) having computer-executable instructions thatcollectively make up the transform function 121 as described herein.Thus, a first computer-readable medium has computer-executableinstructions for estimating the positions of the sources 16 andreceivers 21 using the navigation data 112, the operating states 116,and the environmental data 118. A second computer-readable medium hascomputer-executable instructions for determining optimum tracks for thesources 16 and receivers 21 using a portion of the input data 110 thatincludes at least the survey design data 120. A third computer-readablemedium has computer-executable instructions for calculating drivecommands for at least two of the spread control elements using at leastone the determined optimum tracks. These computer-readable mediums maybe combined or consolidated in a manner that is well known in the art,such as by placing the respective computer-executable instructions on asingle compact disk. The drive commands preferably account for timedelays in the response of the spread 10, as previously described.

Validation

An important component of the inventive spread control system isindependent validation (steps 127, 128) of the optimum tracks calculatedat step 124 and the drive commands calculated at step 126. Validation isimportant for several reasons, including:

-   -   1. to ensure the safety of the vessel 11 and other vessels in        the neighborhood;    -   2. to ensure that the spread control elements are operated        within manufacturers' tolerances; and    -   3. to ensure that individual failures are prevented from        propagating into costly equipment loss or damage.

Validation occurs at several levels and modes of the operation. Thevarious levels of validation include:

-   -   1. internal consistency of optimum track for all spread elements        within the spread 10;        -   a. predicted position changes within spread relative            proximity limit?        -   b. predicted velocity changes within spread relative            velocity limit?        -   c. predicted position changes within obstruction proximity            limit?        -   d. predicted velocity changes within obstruction relative            velocity limit?        -   e. predicted resultant tension within allowed tension            limits?    -   2. drive command parameters to be within limits appropriate for        particular modes of operation;        -   a. predicted resultant tension within allowed tension            limits?        -   b. all deflector angles of attack within stall limits?        -   c. all wing angles of attack within stall limits?        -   d. all vessel control apparatus within limits to restrict            heading change?    -   3. drive commands sent to devices with a high degree of coupling        to be checked for antagonism (for example, steering adjacent        streamers towards each other when they are already too close).

The modes of operation taken into account include:

-   -   1. straight-line production—characterized by low rates-of-change        of drive command parameters;    -   2. non-straight-line production—characterized by medium        rates-of-change;    -   3. turn during non-production—characterized by higher        rates-of-change;    -   4. deployment—characterized by high rates-of-change and some        limits not being observed; and    -   5. emergency—characterized by the fewest controls on drive        command parameters.

Validation further includes the checking of system limits, such as:

-   -   1. spread control element control limits not exceeded—for        example, end-stops on rudder;    -   2. environment sensor limits not exceeded—for example, towing        tension, diving streamer in shallow area;    -   3. rate of response to spread control element control setting        changes—for example, a steering change may have little effect on        a vessel if the wind is opposing it, but a large effect if the        wind is assisting it; and    -   4. position of a spread component not outside an acceptable area        (or inside an unacceptable one).

In normal operation, the validation will allow the checked controlsettings to proceed to the respective spread control element controllerafter positive validation. If the request is rejected, a warning messageis sent to the Operator and the request is blocked. The Operator wouldthen take overriding action as necessary to control and correct thesituation.

Minimum Coupling Model Example

A particular embodiment of the present invention that employs ahydrodynamic model within the transform function will now be describedwith reference to FIGS. 3-13. The spread control elements will becontrolled as independently as possible, or can be coordinated manuallyby an operator. Within the vessel reference frame, the spread controlelements are treated as independently as possible. In another embodimentof the invention, all spread control elements are controlled by a highlyintegrated control system with a comprehensive coupled model.

Vessel Steering

The role vessel steering plays in spread control is to position thetowpoints for the towed spread bodies such that they can maneuver intoan optimum position for each seismic shot. The reactive characteristicsof a survey vessel (i.e., its performance specifications) must be partof the algorithm that plans the vessel steering. When computing thedistance into the future to project the vessel path, knowledge of forcesthat will be encountered by the vessel 11 along this future path must beconsidered.

The amount of steering needed to control source and receiver positionscan be influenced by the base survey. If the base survey was conductedto optimize coverage, current forces with a cross-line component mayhave caused the spread to oscillate along the shooting line. Especiallyin conventional survey configurations, vessel steering is the most usualmethod to reduce infill. If, however, the base survey was conducted tooptimize the probability of successfully repeating the same seismicenergy ray paths in future surveys, the vessel might have steeredstraight along the pre-plot line.

Depending on the survey objective, one or more of the spread componentsshould occupy a target space along the survey line. The vessel path thatwill allow this can be computed using a best-cost map method asdescribed by U.S. Pat. No. 6,629,037. This method is further developedbelow.

Decoupling the Spread Control Elements from Each Other

As stated above, within the vessel reference frame, cross-line forcescan be exerted by the spread control elements being towed. Thus, thevessel must be looked at as being coupled to the towed spread, but thespread control elements can be looked at as if they are independentwithin the vessel reference frame. The coupling to the vessel will beweighted for one or more of the towed spread elements, depending on theobjective of the system. Decoupling of the spread control elementswithin the spread control model is presently believed to provide thebest solution (although others exist) for determining how the elementsinteract and influence the vessel trajectory.

If the spread control elements can control the spread 10 adequately tomeet the positioning objectives with no cross-line contribution from thevessel 11, the spread control elements are, to a large degree,practically and conceptually decoupled from the vessel.

The Source Array

The inline motion of the source arrays 16 is determined by the vessel11. The distance cross-line the source arrays can travel is constrainedby the towing configuration (e.g., ropes or lead-ins 20). If the sourcearrays can be steered within this cross-line constraint corridor, andthe target is within this corridor, optimum source positioning can beachieved. Several mechanisms for positioning within this corridor arepossible. These include:

-   -   1. extra parallel gun strings that can be dynamically combined        to give the source array according to their proximity to the        desired cross-line position;    -   2. winch systems that control the towing configuration relative        to outer streamer tow ropes; and    -   3. deflectors on the source array with controllable angles of        attack.

By these mechanisms, the source position can be steered cross-line toget the best possible position with, depending on the mechanism in use,little or no regard for any other spread control element. This assumesthe vessel 11 hasn't deviated cross-line from the pre-plot line by morethan the source steering devices 17 can correct for, (i.e., isdecoupled) and the cross-line forces on the source arrays 16 can becountered by the steering device 17 in use.

Steerable Streamer Front End Deflector (SFED)

The SFED 22 developed for time-lapse applications can drive the frontend of the streamers 18 cross-line. Depending on the length of the leadin cables 20, cross-line motion will change the inline component of theindividual streamers, giving a skew to the collective front end,commonly called streamer front end skew. Here there is a couplingbetween inline and cross-line, but it's slight.

There are several reasons for steering the front end of the streamers18. One is to prevent the outer streamer front ends from rotating,(front end skew). One of the most evident causes of front end skew isvessel steering. Also, SFED steering can be used to shift the front ofthe streamers cross-line. Finally, SFEDs control streamer separation.All of these steering objectives contribute to positioning the streamerfront end, which is the reference point for the streamer steeringalgorithm described below.

Streamer Steering Devices (SSDs)

The SSD global controller has several modes with the objective todeliver a demanded individual or collective streamer shape. Constantfeather and constant separation are two examples.

Feather Mode

In feather mode, the SSD global controller uses the front end of thestreamer as a reference point (srp), effectively an origin, from whichan ideal streamer shape is computed relative to some referencedirection, for example, the pre-plot line direction. One case of thisshape is a streamer 18 with constant feather, i.e., substantially theentire streamer has the same feather as seen in FIG. 3. Thus a desiredfeather of zero degrees is obtained by steering towards a virtualstreamer computed by extending a straight back from the front referencepoint of the streamer and parallel to some reference direction like thepre-plot line direction.

Here is a coupling or cooperation between the SFED and the SSD globalcontroller. As stated above, steering to get the correct reference pointfor the streamer front end is another objective of SFED steering.

Constant Separation Mode

This mode, shown in FIG. 4, functions by comparing the distance from theSSD on the adjacent streamer to the desired separation. The SSDsfunction to keep all streamers a user-entered distance apart. Duringperiods of skew, the cross-line component of the distance is resolvedfor comparison.

Limits to Steering

Anticipation of the forces ahead, particularly the cross-line forces dueto currents, will dictate what drive commands (e.g., steering) give thebest outcome. However, steering the spread control elements may notovercome all cross-line forces the spread might encounter. When thesteering limit of the spread is reached, the transform function 121optimizes the shape of the spread 10 to fit the survey objective(s). Theoptimal streamer shape might be straight with a desired feather angle(see, e.g., FIG. 3), it might have local feather angles defined bysegments along the streamer to achieve a best fit for a prior streamersurvey shape (see, e.g., FIG. 5), or the streamers might be evenlyspaced (see, e.g., FIG. 4) to allow better trace interpolation in theseismic data processing step.

Current Model within the Transform Function

The same simple current model described for the source array feather andillustrated in FIG. 9 also applies to the streamers 18. As long as thecurrent is constant to some degree over the length of the streamer, thesame model and accompanying ability to predict future natural featherangles is valid. The calibrated spread model ability to achieve steeredfeather can be added to the natural feather to get a desired streamershape.

Force Model within the Transform Function

Desirable hydrodynamic force models may be derived from the teachingsof: P. P. Krail and H. Brys, “The Shape of a Marine Streamer in aCross-Current”, Vol. 54, No. 3 of the Journal of the Society ofExploitation Geophysicists; Ann P Dowling, “The Dynamics of TowedFlexible Cylinders,” Part 1: Neutrally Buoyant Elements, and Part 2:Negatively Buoyant Elements, 187 Journal of Fluid Mechanics pp. 507-532,533-571 (1988); C. M. Ablow and S. Schechter, “Numerical Simulation ofUndersea Cable Dynamics,” Ocean Engineering, 10: 443-457 (1983). Thealgorithms used to predict the streamer behavior within the transformfunction are based on these teachings and give a significant improvementto streamer behavioral prediction when combined with models of spreadcontrol elements coupled with the streamer. An example of a commercialimplementation of streamer cable shapes resulting from theabove-referenced force model theory, and including SSDs, is Orcina'sOrcaFlex™ cable modeling software.

Track Optimization Formulas

These formulas are based on optimizing the differences between desiredand actual positions and/or shapes along a shooting line for theindividual spread bodies, and assume decoupling as described above. Oneof the salient constraints is the reaction time of the various steeringdevices. Reaction times can be measured with any frequency, depending onthe navigation solution rate. Reaction times are practically on theorder of the shots, i.e., typically tens of seconds. The steeringcontrol will thus plan several shots ahead and is likely to be vesseland spread dependent.

Further, in the calibration computations, spread control elementsreaction times, will be estimated based on the recent history ofreaction times, learned from the navigation data input 112. Thesereaction time estimates for various spread control elements will then beused in the optimum track estimation 124 to facilitate the calculationof realistic drive commands (at 126).

Vessel Trajectory

The vessel path can be planned to keep the tow point for the towedspread control elements within the constraint corridor that allows thesteering available in the spread to achieve the target shape and track.Thus given a particular desired shape that can be achieved by the towedspread control elements, an optimum track for the towpoints is estimatedthat gives an adequate cross-line component relative to the optimumtrack for the towed spread. The optimum track for the towed spread isderived from the objective of the present phase of the operation. It maybe a time lapse survey and the objective might be for a certain offsetgroup to re-occupy the same track as it did in the base survey. It mightbe close pass of a production platform and the objective is for theclosest towed object, the streamer end for example, to keep a distanceof 50 meters from the platform. With this track realized, the towedspread is decoupled in the sense that it can maneuver adequately withinthe vessel reference frame.

The algorithm that allows this towpoint track is a best-cost map methodas described by U.S. Pat. No. 6,629,037. Here a particular element ofthe spread, an offset group of the streamers or the center of the sourcearray for example, is given a higher weight in the best track search forthe vessel. The coupling model can be for example a straight linebetween the vessel towpoint and the highly weighted spread element(s),and will be as accurate as the ability of the spread control system torealize that shape. The goal of the vessel towpoint track estimate willbe to give the cross-line shift between the towpoint and the criticalspread element(s). The track can be recomputed as often as thecomputational power available will allow. The re-computation of thetrack may not be required at a high frequency since the vessel towpointin the area relative reference frame, and towed spread element relativeto the vessel towpoint, change slowly in the cross-line direction duringa typical survey.

This track estimate can be computed in the planning stage with apre-survey estimate of spread body steered feather and survey objectivespread target set. This planned track can be used in the algorithmfollowing to anticipate the amount of steering that might be requiredfor a particular survey.

Once the track is computed, with a start point at the present vesseltowpoint position, a plan for realizing this track is computed. Here theresponse time of the vessel is the limiting factor. The best costcomputed track must be realized in a stable way that minimizesover-steering.

A smooth vessel track to occupy the optimum track can be computed withthe following algorithm. The area relative coordinate frame is used forthis development. In this reference frame, y is the inline axis and x isthe cross-line axis.

Thus:

ΔVes _(x) =X _(spi) −X _(spi+n)  Eqn 2

ΔVes _(y) =Y _(spi) −Y _(spi+n)  Eqn 3

where:

-   -   sp_(i) is shotpoint number i.    -   sp_(i+n) is shotpoint number i plus n shots into the future.    -   ΔVes_(x) is the difference between the current vessel crossline        coordinate and the crossline coordinate n shots ahead from the        previous survey.    -   ΔVes_(v) is the shotpoint distance.

The steering model as shown in FIG. 6 is a straight line:

(ΔVes _(x))_(steered) =m(ΔVes _(y))_(steered) ⁺β  Eqn 4

where:

m is the estimated slope or crossline change to steer toward with inlinemotion.

β is the current crossline coordinate.

The steering plan is based on the best fit line to the best cost trackestimate described above. The observation equations are written inmatrix notation:

A _(x) =b+v  Eqn 5

where:

$\begin{matrix}{A = \begin{bmatrix}1 \\1 \\1 \\1\end{bmatrix}} & {{Eqn}\mspace{14mu} 6}\end{matrix}$

x=m, the slope estimate

b=(ΔVes_(x)/ΔVes_(y)) measured

v the residual of the fit.

The least-squares solution to this equation can be written:

x=(A ^(t) A)⁻¹ A ^(t) b  Eqn 7

The simple A matrix shown above will have more significance in theweighted solution. The example shown projects four shot points forward,as represented by the four observation equations indicated in the Amatrix.

In order to constrain the amount of steering caused by one or morefuture shot points that were the result of poor steering and notindicative of the trend, we can introduce dynamic weighting. Theweighted L2 solution is written:

x=(A ^(t) PA)⁻¹ A ^(t) Pb  Eqn 8

where:

$\begin{matrix}{P = \begin{bmatrix}{P_{ii}000} \\{0P_{ii}00} \\{00P_{ii}0} \\{000P_{ii}}\end{bmatrix}} & {{Eqn}\mspace{14mu} 9}\end{matrix}$

at iteration (ii=1).The residuals are computed from iteration (ii=1) with:

v _((ii=1)) =Ax−b  Eqn 10

Each individual residual is compared with the standard deviation of allthe residuals. The largest residual that is also greater than a limitthat would cause excessive heading changes:

|v|_(ii)>2σ_(ii) for example, can be downweighted as some function ofthe residual and the line fit again with:

P _((ii+1)) =f(|v|) if |v| _(ii)>2σ_(ii)  Eqn 11

and

P _((ii+1)) P _(ii) if |v| _(ii)>2σ_(ii)  Eqn 12

Re-weightings continue until the heading change is acceptable to theobjectives set for the towed spread control objects.

Alternatively, the value n can be increased until the long-term trendgives a heading change that is acceptable. A minimum number, dependingon the vessel response possible, is used. If the resulting headingchange is larger than the limit, the line can be recomputed based on abest fit for n+1 and so on until the heading change is below the limit.

This computation of vessel track is repeated for each shot cycle basedon the actual position occupied at shot time. In a pre-surveyapplication, the position for each shot is taken to be the position thatwould have been reached after traveling along the straight line untilthe next shot location was reached. At each shot a new line, giving anew heading is followed.

Studying the best steering strategy pre-survey, in the planning stage,will allow a better understanding of how far ahead to extend the linefit through trial and error. In addition, it will give the navigator anidea of the approximate steering they might encounter. It can then berecomputed online to give the steering required in situ, but withconstraints and difficult periods identified in the survey design phase.

The maximum heading change will be determined by a number ofconsiderations including;

-   -   1. the ability of the vessel 11 to move the tow points of the        spread elements cross-line;    -   2. towed spread ability to move cross-line in one shot (one        spread control element will limit the rest);    -   3. weighting based on how many shots will be out of spec (i.e.,        will deviate from the optimal track) with longer look ahead        projections;    -   4. the value of the zone of the survey, where some areas or        zones of the total survey area might by less interesting than        others due to the subsurface targets believed to exist there;        and    -   5. spread element weighting, the re-computation of the best cost        track

The normal vessel shooting speed gives adequate device-relative waterflow to operate passive steering devices such as SFEDs. Forces resultingfrom changes in the towed spread control elements must not have asignificant impact on the vessel heading. As an example consider arapidly changing significantly different tension from port to starboardtowpoints that could cause the vessel to crab. Thus tension should bemonitored at the towpoints to assure it's not excessive and also inbalance in relation to the vessel.

FIG. 6 illustrates a best-fit straight line according to a look-aheadprojection of 4 shot points, wherein the residual projections arerecomputed based on the location after each shot point. The number ofshots in the future that define the line determine how drastic thesteering will be, with one shot point being the most drastic.

FIG. 7 shows a combination of successive look-ahead best-fit straightlines like that of FIG. 6. The resulting segmented look-ahead path is asmoother, and more realistic, vessel trajectory compared to the priorsurvey line. Those skilled in art will appreciate that theabove-described straight line path is but one model that may be used toadvantage in accordance with the present invention.

The Source

The behavior of the source arrays can be measured as a function of aheading change. The source arrays largely follow the vessel track butcan also be shifted cross-line by current and, to a lesser degree, wind.A source steering device 17 can only compensate by a limited amount forcross-line shifts. Once the steering limit is exceeded, vessel steeringis the only tool left to put the source in the desired position.

Source Calibration for Steering

With a calibrated model of source array behavior relative to the vessel11, the source array position can be predicted relative to the vesselalong a survey line. Factors that can be added to the prediction modelare expected currents and wind along the line.

Measures of inline and cross-line change are provided by GPS receiverson the source array floats. The locating of GPS receivers on the sourcearray gives the rate of change in gun string array coordinates withrespect to heading changes. For each new vessel heading, the time andtrajectory taken by the source array 16 before it stabilizes behind thevessel 11 is the measure of the system reaction.

Current Effect on Source Array and Current Calibration

In addition to the effect of vessel heading changes, source behavior dueto any other relevant force such as wind, can be measured. Acurrent-induced source cross-line shift can be expressed in terms ofsource feather angle and described by the simple comparison illustratedin FIGS. 8A and 8B. The resultant from the vessel movement and the watercurrent vectors in the area relative reference frame gives the sourcefeather angle. Thus given the value of R (the distance from the sourcetow point on the vessel to any point on the gun string), and the featherangle, the source array coordinates can be predicted. Since the relationbetween feather angle and current is known, in the absence ofsignificant cross-line wind, measured feather angle gives the currentdirection and can be used to calibrate any source of currentinformation.

FIG. 8B shows a schematic representation of current and vessel velocityvector resolution. Air currents with a cross-line component, wind,against the gun array surface floats will shift the source arraycross-line if the force exerted is large enough. In order to estimatethe cross-line displacement, an aerodynamic model of the float surfacearea must be used.

SFEDs

The SFEDs can react to the position estimates of the streamer headreference points as they are used today to drive SSD feather mode in theglobal controller. The SFEDs' expected proximity to the pre-plot or basesurvey coordinates based on estimated vessel tow point position will bethe basis for calculating the SFED drive (steering) commands. The SFEDs'objective will be to drive the head of the streamers into the optimumposition to allow the SSDs to locate the streamer length optimally. Inaddition, the SFEDs should stabilize the front end of the streamers,which is especially important for the SSDs in feather mode since thefeather is computed from the front end reference point.

As with vessel steering, drive commands are based on reaction time ofthe SFEDs. SFED reaction time will be measured continually during thesurvey and fed back to the transform function to gauge the look-aheadperiod. The same model as described above for vessel steering—a straightline fit some number of shot points ahead—is an example of how drivecommands can be computed for the SFEDs.

A nominal orientation, perpendicular to the pre-plot for example, mightbe the desired orientation of the spread during the base survey. Arecording of the base survey orientation containing each shot of thebase survey is replayed during the repeat survey to give the SFED targetorientation. In addition to orientation, cross-line positioning can beachieved by the SFED.

Base survey positions for the streamer front ends will be used todetermine what streamer front end orientation gives the optimum repeatpositions. Repeat or time lapse survey spreads may have the same numberor more streamers than the base survey. Time lapse spreads may have thesame or denser streamer separation distances as well. In all cases, theobjective would be to match streamer front end coordinates in cross-lineand inline positions.

FIG. 9 shows a correction or change in streamer front end by theexecution of drive commands delivered to the SFEDs. The correctionresults in the streamer front end being offset at an angle to the coursemade good, overcoming the current-induced crab angle θ.

In addition to orientation, a mean cross-line coordinate, in the vesselrelative coordinate frame can be computed for steering purposes. Thismeans the streamer front end can be used as a target. Accordingly, FIG.10 shows the streamer front end centers being fitted to a desiredsteering track.

The behavior of the spread front end with respect to the vessel headingcan be estimated in a similar way as was described above for the sourcearray. The rotation of the tow points can be either measured directly bylocating GPS antennas on them or indirectly through the change in thestreamer front end coordinate estimate change as a function of vesselheading changes. As in the case of the source array, current larger thanthe SFED ability to steer will drive the front end out of equilibrium.Further, the SSDs can assist the SFEDs by anticipating the change in thevessel heading and the estimate of impact caused by the change ofheading on the streamer front end.

Steerable Tailbuoys

Since the streamers are not controlled after the last SSD, the tailbuoyswill be useful to bring the tail ends into place. Positioning in feathermode will be the continuation of the straight feather line made by theSSDs, parallel to the full length of the streamer. To obtain longoffsets, steerable tailbuoys will be useful to get the tail end of thestreamer on target.

Optimal Feather Angle Estimation

Decoupling of the source array track, and assuming a straight streamer,fitting a line to the set of coordinates occupied by the previousreceivers along a streamer is a solution that decouples the sourcecontrol from the streamer control. The vessel and SFEDs will cooperateto get the streamer front end, called the streamer reference point(srp), into position. Both in pre-survey planning and real time, thecoordinate prediction of the srps will be used by the global controlleras the start point of the streamer. From this predicted streamer startpoint, a straight line least squares fit to the targets along the basesurvey streamers can be computed for each shot. This fit will give anoptimum matching streamer feather angle.

A global or individual feather angle can be computed for the streamers.The global controller will instruct the SSDs to assume this featherangle for steering. The feather angles demanded must not change morerapidly from one shot to the next than a specified limit. This can belimited as in the straight line fit for the vessel trajectory byisolating the target(s) responsible for any demand in rapid featherangle change. This computation can be done pre-survey and the featherangle speed limited, or in other words, the outlier shots downweighted.

Optimal Feather Angle for all Streamers

The optimal feather angle can be computed based on the base surveycoordinates. Again the quantities driving the optimal feather anglechanges demanded by the transform function are the residuals formed bydifferencing the actual and desired receiver coordinates.

For each shot, given the predicted coordinates of the srp, there is aline that starts at srp(x,y) and is projected sternwards a distanceR_(str) equal to the streamer length, at some feather angle with respectto a reference direction such as the shooting direction, that fits bestin some sense, (such as least squares) the set of receiver coordinatesthat are to be reoccupied during a time-lapse survey.

The vertical axis is perpendicular to the reference directions, and thesrps are on this line. The horizontal axis for convenience passesthrough the vessel reference point, located mid vessel, and is notrelated to the vessel heading except when the vessel is perfectlyparallel with reference direction, the shooting direction for example.The origin is at the intersection of the two axes. All srps can benormalized to the system origin to form an observation equation thatgives a common slope. As illustrated in FIGS. 11 and 12, the commonslope of the “best fitting” lines BF for all base survey streamers 18can be estimated and converted to a common feather angle Φ for allstreamers at each shot. The srps will give the y intercepts for theselines.

The conversion from slope to feather angle is the conversion fromCartesian to polar coordinates. If for any Rec (x,y) pair on the bestfit lines, x/y=m, then:

Arctan(m)=feather angle.  Eqn 13

For any receiver i (Rec_(i)(x,y)) on any streamer j, given an alongdistance relative to some common origin, a cross line value, normalizedby the cross line component of the srp for that streamer, an observationcan be formed to yield a slope.

m _(i)=(x _(i) −b _(j))/y _(i)  Eqn 14

These observations can be formulated to give the observation equationsas shown in Equation. 5, (i.e., Ax=b+v) where the number of observationequations is equal to the number of receivers on all streamers, n=(j*i).

$\begin{matrix}{{A = \begin{bmatrix}1 \\1 \\1 \\\vdots \\n\end{bmatrix}},{x = m},{b = \begin{bmatrix}{\left( {x_{1} - b_{1}} \right)\text{/}y_{1}} \\{\left( {x_{2} - b_{1}} \right)\text{/}y_{2}} \\{\left( {x_{3} - b_{1}} \right)\text{/}y_{3}} \\\vdots \\{\left( {x_{i} - b_{1}} \right)\text{/}y_{i}} \\{\left( {x_{1} - b_{2}} \right)\text{/}y_{1}} \\{\left( {x_{2} - b_{2}} \right)\text{/}y_{2}} \\\vdots \\{\left( {x_{i} - b_{2}} \right)\text{/}y_{i}} \\\vdots \\{\left( {x_{1} - b_{j}} \right)\text{/}y_{1}} \\{\left( {x_{2} - b_{j}} \right)\text{/}y_{2}} \\\vdots \\{\left( {x_{i} - b_{j}} \right)\text{/}y_{i}}\end{bmatrix}},{{{and}\mspace{14mu} v} = \begin{bmatrix}{x_{1} - {\hat{x}}_{1}} \\{x_{2} - {\hat{x}}_{2}} \\{x_{3} - {\hat{x}}_{3}} \\\vdots \\{x_{n} - {\hat{x}}_{n}}\end{bmatrix}}} & {{{Eqns}.\mspace{14mu} 15}\text{-}18}\end{matrix}$

The simple solution to Equation 6 is also Equation 8, and the weightedleast-squares solution is Equation 9.

Again, as in Equation 9, the slope and thus feather angle change can beconstrained by downweighting very large observations values. Further,the slope estimate can be constrained to favor any offset group bygiving that group a higher weight relative to less important offsetgroups.

The application of this estimation is advantageous for reducing infillin a near real-time situation along straight pre-plot lines wherecurrents are present, but is perhaps most useful for reoccupyingreceiver positions shot on a previous survey where there was difficultyobtaining coverage by following the straight pre-plot line. While it'snot currently common practice for the srps to follow a non-straightpre-plot line in favor of a track that gives the best repeat positions,(i.e., time lapse surveying), this estimation process will makerepeating receiver positions easier.

Optimal Feather Angle for Individual Streamers

FIG. 13 illustrates that the above-described estimation of the optimumslope and thus feather for all streamers is applicable for estimating anoptimum slope for individual streamers 18 (see best fit lines BF₁ andBF₂ for streamers S₁ and S₂). Optimizing the slope with a “best fit”line for each individual streamer has the advantage of giving a betterfit to the base survey receiver coordinates. This advantage brings withit some level of complication in that the feather angle from streamer tostreamer cannot be too different before the risk of collision occurs.Since streamers should not become dangerously non-parallel even withoutthe aid of steering devices in a conventional spread, it reasonable topredict that the risk will be infrequent.

The simplest way to deal with the risk is simply to use the estimatedchange in feather for the individual streamers to give the relativeproximity and velocity those changes will give. If predefined limits areexceeded, some weighting criteria for coordinating the individualfeathers is needed. Since the base survey coordinates can be madeavailable known before the real time danger is encountered, situationslikely to approach the risk avoidance limit can be managed before therisk is encountered. In addition, software checks in real time are usedto eliminate the risk of streamer collision due to conflicting featherangles for individual streamers.

Current and Wind

In this discussion the use of the term “natural feather” is tocharacterize the combined effect of current and wind on surface objectsto move them cross-line. Vessel heading, source array feather, andtail-buoy feather, in the absence of cross-line steering, occur due tosurface current and wind (swell also). When these long period motionsare observed in the position estimates, a trend is identified. If thetrend is spatially short in relation to the spread extent, a local trendis identified and can be anticipated by the spread control elementsfollowing. If the trend is persistent in time, it can be remembered bythe system in space and expected to recur when the spread passes thisarea of cross-line force.

Calibration

Many measures of a quantity can be combined to get a better estimate ofthat quantity than any one alone. This principle will be applied in thespread control system described here. The fact that the spread covers alarge horizontal space and can be equipped with measuring devicesthrough its vertical extent is an opportunity to measure quantitiesrelevant to spread control over significant portions of time and space.In addition, the error states of a measuring device can be estimatedbased on additional measurements from other independent sources.

Current Meter Sources of Error

Calibration of measures that contribute to current speed and directionwill be conducted in real time. Hull mounted current meters often giveinaccurate measures of current depending on their location in relationto the propeller wash and other interference. Further, they reportcurrent at the depth they are located and this current may not apply toeither the surface or streamer depth.

Current Meter Calibration with a Least Squares Fitting Model

As described previously, the resultant direction of the current on thetowed source array can be measured by the response of in-sea equipment.Data from current meter devices located at the depth of this in-seaequipment can be compared to force model-computed values, found throughthe feather angles computed based on coordinate estimates, and. T, thedifference between the observed current meter readings and computedcurrent can be fit to an error model within the transform function.

The best model to use for this relation will depend on the instrumenterror characteristics and other sources of error present. Although thereare nearly an infinite number of mathematical functions that might bebest, we can use a simple linear model as an example.

The line model has a constant component that is analogous to a bias anda scale component that can describe a change with respect to somevariable like current magnitude or in-sea equipment response time.Residuals in computed and measured current are fit to the line model.

Measures of current that affect surface devices such as the vessel,source arrays, SFEDs, and tailbuoys can be combined to get the bestestimate of surface current. Besides current meters, the vessel headingand source arrays, corrected for winds and waves in a force model, cangive information about surface currents. Trends in cross-line motion notexplained by either vessel motion or device steering can also be used asmeasures of cross-line current on the surface.

At streamer depth, current measuring devices along the streamer give anindication of the current there. In straight non-assisted towing, as thespread passes through a zone of current, each streamer mounted currentmeter should give the same measure of current at any given point alongits trajectory as the meter that preceded it except for any time varyingchanges occurring between subsequent passage. Again, fitting a functionto describe a trend of change, (time varying assuming the currentspatial extent is larger than the horizontal and vertical deviation ofsubsequent streamer mounted devices), will show a bias caused by any onecurrent meter, compared to all others. In cases where current spatialextent is less than the spread size, local current trends can beestimated.

Cross-line Speed Calibration

In real time, the cross-line response of steering devices can beestimated. Time taken to reach the target feather given a feather changecommand reveals the response time of the individual spread controlelements to drive commands in the real time environment they occur. Thisinformation will be fed back to the computations of optimum drivecommands.

For example, measurement of the cross-line component of vessel speed vs.heading change can be fit to a function that describes the relation. Themathematical description of the small changes expected while steeringalong a time lapse survey line are likely not complicated due to thesmall range over which the function is relevant. The sequentialestimation formulae can be applied to get an update of steering deviceresponse time as frequently as position updates are available.

Tension Calibration

Tension measurements may be calibrated against inline water velocitymeasurements, which are related. When tension expected from thehydrodynamic drag model disagree with those measured, either the tensionmeasurement or a parameter in the hydrodynamic model are the cause ofthe predicted residual. Parameters such as water velocity and body dragcoefficient, based on the effective surface area of the body beingdragged, give the tension expectation. Correcting these to give improvedagreement with accurate tension meters will give a better tunedhydrodynamic model.

Steering Body Calibration

The navigation solution contributes to improved hydrodynamic modeling.Knowledge of the orientation of the SSD bodies and the current vectorgive the force available for steering. Such orientation can be computedbased on the navigation solution. With this information, SSD wing angleof attack can be translated to a more accurate force vector givingimproved control of the spread, as described in International PatentApplication No. WO 00/20895.

Validation

When a set of optimal shot point target coordinates and/or streamershape changes are estimated, a safety check is made to determine if acollision between spread elements is probable. If the check determinesthe computed optimization is above the target risk limit, this isreported to the user online. The user is then offered a set ofalternative steering constraint choices to change that will give adifferent outcome to the optimization computation.

After the optimal shot point target coordinates and/or streamer shapechanges are deemed acceptable, they are used in the spread model togenerate optimal spread control element drive commands. These commandsare then simulated within the spread model to give the operating states.These operating states are also checked against limits beyond whichfailures may occur. If it is determined that any of the limits must beexceeded to realize the optimal shot point target coordinates and/orstreamer shape changes desired, the limiting spread control element isconstrained and an alternate set of drive commands is computed. Thenumber of alternative that can be tried is dependent of computationalspeed available within the operating update cycle. In parallel, analternative set of optimal shot point target coordinates and/or streamershape changes can be computed that will require less of the offendingspread control element to give an acceptable set of drive commands. Ifno safe set of drive commands is available, the online operator assumesmanual control through an intelligent GUI with guidance based on spreadelement operating state information and spread element motion historyand prediction clearly presented.

Spread Control Element Relative Proximity Check.

Position estimate differences larger than defined limits for allseparately controlled bodies at all points of the body where there is aposition estimate available will result in the calculation of differentdrive commands. Limits for proximity are based on the quality of theposition estimate.

Spread Control Element Relative Velocity Check.

All point-relative velocity estimates for all points on separatelycontrolled spread bodies must be less than the limit. The limit is basedon the time to next check and the quality of the velocity estimate. Ifduring the time to next check a collision or near collision will occur,drive commands to avoid collision is required. The limit is a functionof the error estimate of the velocity.

Spread Control Element Obstruction Proximity Check.

The distance between the position estimate of any point in the spreadrelative to all obstructions must be less than some limit. The limit isa function of the quality of the position estimate.

Spread Control Element Obstruction Relative Velocity Check.

Velocity estimates cannot result in a proximity larger than a limit overthe time before the next velocity estimate cycle. This limit is afunction of velocity estimate quality.

Mechanical Integrity Check.

Among the mechanical integrity checks are: no cable tensions being outof bounds; and no steering device wing angles approaching stall.

It will be understood from the foregoing description that variousmodifications and changes may be made in the preferred and alternativeembodiments of the present invention without departing from its truespirit.

This description is intended for purposes of illustration only andshould not be construed in a limiting sense. The scope of this inventionshould be determined only by the language of the claims that follow. Theterm “comprising” within the claims is intended to mean “including atleast” such that the recited listing of elements in a claim are an opengroup. “A,” “an” and other singular terms are intended to include theplural forms thereof unless specifically excluded.

What is claimed is:
 1. A method comprising: collecting input data from aseismic survey spread having a plurality of spread control elements, aplurality of navigation nodes, and a plurality of seismic sources andreceivers, the plurality of spread control elements having at least aseismic source control element and a streamer control element, both ofwhich are configured to be towed by a vessel, the input data including:navigation data for the navigation nodes; operating states from sensorsassociated with the spread control elements; environmental data for thesurvey; and survey design data; estimating positions of the seismicsources and receivers using the navigation data, the operating states,the environmental data, or combinations thereof; determining optimumtracks for the seismic sources and receivers using the estimatedpositions and a portion of the input data that includes the surveydesign data; and calculating drive commands using the determined optimumtracks for controlling the seismic source control element and thestreamer control element configured to be towed by the vessel.
 2. Themethod of claim 1, wherein the navigation data, the operating states,the environmental data and the survey design data are input to atransform function to perform the estimating, determining, andcalculating steps.
 3. The method of claim 2, wherein the positions areestimated according to a spread model within the transform function, andthe optimum tracks are input to the spread model for calculation of thedrive commands.
 4. The method of claim 3, wherein the spread modelcalculates a first set of estimated positions using input that includesat least the operating states and the environmental data, the navigationdata includes a second set of estimated positions, and the first andsecond set of estimated positions are combined with the transformfunction to produce the estimated seismic source and receiver positionsand predicted residuals.
 5. The method of claim 4, wherein the predictedresiduals are used to estimate a set of parameters that characterize thespread model, and the spread model parameters are used to calibrate thespread model.
 6. The method of claim 4, wherein the predicted residualsare used to estimate error states associated with one or more sensorsthat measure the environmental data.
 7. The method of claim 2, whereinthe optimum tracks are determined according to a weighting functionwithin the transform function, wherein the weighting function receivesas inputs the survey design data and the estimated positions.
 8. Themethod of claim 1, further comprising automatically validating thecalculated drive commands and delivering the validated drive commands tothe spread control elements, whereby a desirable survey objective may beattained.
 9. The method of claim 1, wherein the drive commands includecommands for controlling at least vessel propeller, vessel thruster,spread component steering device, or vessel cable winch.
 10. The methodof claim 1, wherein the sensors associated with the spread controlelements include one or more sensor types of tension, water flow rate,inclination, orientation, acceleration, or combinations thereof.
 11. Themethod of claim 1, wherein the collected environmental data includes oneor more data types of current, salinity, temperature, pressure, speed ofsound, wave height, wave frequency, wind speed, and wind direction. 12.The method of claim 1, wherein the survey design data is selected fromspread tracks, performance specifications, and survey objectives,wherein the performance specifications are selected from drag andmaneuvering characteristics for the vessel, steerable cable devices,steerable seismic source devices, and deflectors, drag characteristicfor the towed cables, seismic sources, and floatation devices, and winchoperating characteristics.
 13. The method of claim 1, wherein the surveydesign data includes one or more data types of area, depth, arearotation or shooting orientation, line coordinates, required coverage,local constraints, optimizing factors and historical data.
 14. Themethod of claim 54, wherein the operator input data includes spreadparameter settings and environmental data, and wherein the pre-surveydata includes environmental sensor data.
 15. The method of claim 54,wherein the real-time survey data includes one or more data types ofcable tension, water flow rate, inclination, orientation, acceleration,velocity, position, spread control element setting, environmental data,seismic signal and noise data, and operator input.
 16. The method ofclaim 54, wherein the simulated survey data includes one or more datatypes of simulated pre-survey, simulated operator input, simulatedcurrent survey, simulated near-real time survey, simulated real-timesurvey, and simulated environmental data.
 17. The method of claim 13,wherein the collected input data further includes raw seismic sensordata, and using the raw seismic sensor data to produce qualityindicators for the estimated positions, the quality indicators selectedfrom binning datasets, absolute noise data, signal-to-noise ratios, andseismic signal frequency content.
 18. The method of claim 3, wherein thespread model is a hydrodynamic force model of the spread components, apure stochastic model of the spread components, employing one of theL-norm fitting criteria, or a neural network.
 19. The method of claim18, wherein the force model contains marine current data.
 20. The methodof claim 3, wherein the spread model is a pure stochastic model of thespread components.
 21. The method of claim 3, wherein the spread modelemploys one of the L-norm fitting criteria.
 22. The method claim 3,wherein the spread model is a neural network.
 23. A system comprising: aseismic survey spread having a plurality of spread control elements, aplurality of navigation nodes, and a plurality of seismic sources andreceivers, wherein the plurality of spread control elements comprises aseismic source control element and a streamer control element, both ofwhich are configured to be towed by a vessel; a database for receivinginput data for controlling the seismic survey spread including:navigation data for the navigation nodes; operating states from sensorsassociated with the spread control elements; environmental data for thesurvey; and survey design data; a computer readable medium havingcomputer-executable instructions for estimating positions of the seismicsources and receivers using the navigation data, the operating states,the environmental data, or combinations thereof; a computer readablemedium having computer-executable instructions for determining optimumtracks for the seismic sources and receivers using the estimatedpositions and a portion of the input data that includes the surveydesign data; and a computer readable medium having computer-executableinstructions for calculating drive commands using the determined optimumtracks for controlling the seismic source control element and thestreamer control element configured to be towed by the vessel.
 24. Amethod comprising: towing a plurality of seismic survey spread elementsgenerally behind a vessel, wherein the plurality of seismic surveyspread elements comprises a plurality of spread control elements, andwherein the plurality of spread control elements comprises a seismicsource control element and a streamer control element; providing a setof desired coordinate positions of the seismic source control elementand the streamer control element, wherein the set of desired coordinatepositions is determined using navigation data for a plurality ofnavigation nodes, operating states from sensors, environmental data fora seismic survey, and survey design data; independently measuring a setof actual coordinate positions of the seismic source control element andthe streamer control element; calculating a difference between the setof desired coordinate positions and the set of actual coordinatepositions to form residuals; and using the residuals as set points inone or more controllers to calculate drive commands for controlling theseismic source control element and the streamer control element beingtowed generally behind the vessel.
 25. The method of claim 24 wherein atleast one of the controllers uses a PID correction method.
 26. Themethod of claim 24 further comprising planning a path for the vesselwithin a constraint corridor that allows steering available in thespread control elements to achieve a target shape and a target track forthe seismic survey spread elements.
 27. The method of claim 24 furthercomprising estimating optimum tracks for tow points of the spreadcontrol elements that provide a cross-line component relative to anoptimum track for the spread control elements.
 28. The method of claim24, wherein each of the drive commands is used to control at least oneof position, speed, and heading of the vessel.
 29. The method of claim24, wherein the drive commands include commands for controlling at leastone vessel propeller, vessel thruster, vessel thruster setting, vesselpropeller pitch, vessel propeller rotation speed, vessel rudder angle,or combinations thereof.
 30. The method of claim 24, wherein the spreadcontrol elements comprise a vessel control element, a source controlelement, and a streamer control element in coordination with each other.31. The method of claim 24, wherein the spread control elements comprisea vessel control element and a source control element in coordinationwith each other.
 32. The method of claim 24, wherein the spread controlelements comprise a vessel control element and a streamer controlelement in coordination with each other.
 33. The method of claim 24,wherein the spread control elements comprise a source control elementand a streamer control element in coordination with each other.
 34. Themethod of claim 24, wherein the spread control elements comprise atleast two vessel control elements in coordination with each other. 35.The method of claim 34, wherein one of the at least two vessel controlelements is associated with a first vessel and another of the at leasttwo vessel control elements is associated with a second vessel.
 36. Amethod, comprising: providing a seismic survey spread having one or morevessels and one or more spread control elements, wherein the spreadcontrol elements comprise one or more vessel control elements, one ormore source control elements and one or more streamer control elements,wherein the source control elements and the streamer control elementsare configured to be towed by the one or more vessels; and controllingthe seismic survey spread by coordinating the positioning of the vesselcontrol elements and the source control elements and streamer controlelements configured to be towed by the one or more vessels.
 37. Themethod according to claim 36, wherein coordinating the positioning ofthe vessel control elements, the source control elements and thestreamer control elements comprises: providing a set of desiredcoordinate positions of the spread control elements, wherein the set ofdesired coordinate positions is obtained from one or more data typesselected from operating states from sensors associated with the spreadcontrol elements, environmental data for the survey and survey designdata; independently measuring a set of actual coordinate positions ofthe spread control elements; calculating the difference between the setof desired coordinate positions and the set of actual coordinatepositions to form residuals; and using the residuals as set points inone or more controllers to calculate drive commands for the spreadcontrol elements.
 38. A method, comprising: providing a seismic surveyspread having one or more vessels and one or more spread controlelements, wherein the spread control elements comprise one or morevessel control elements, one or more streamer control elements and oneor more source control elements, wherein the source control elements andthe streamer control elements are configured to be towed by the one ormore vessels; estimating one or more positions of the spread controlelements based on data received from one or more acoustic positioningreceivers and one or more reference points on a seismic survey spreadwith respect to the earth; and controlling the seismic survey spread bycoordinating the positioning of the vessel control elements and thesource control elements configured to be towed by the one or morevessels based on the estimated positions.
 39. A method, comprising:providing a seismic survey spread having one or more vessels and one ormore spread control elements, wherein the spread control elementscomprise one or more vessel control elements, one or more source controlelements and one or more streamer control elements, wherein the sourcecontrol elements and the streamer control elements are configured to betowed by the one or more vessels; and controlling the seismic surveyspread by coordinating the positioning of the streamer control elementsand the source control elements configured to be towed by the one ormore vessels.
 40. A method, comprising: providing a seismic surveyspread having a first vessel and a second vessel, the first vesselhaving a first vessel control element and a first source control elementconfigured to be towed by the first vessel, and the second vessel havinga second vessel control element and a second source control elementconfigured to be towed by the second vessel; estimating one or morepositions of the first vessel control element, the second vessel controlelement, the first source control element and the second source controlelement based on data received from one or more acoustic positioningreceivers and one or more reference points on the seismic survey spreadwith respect to the earth; and controlling the seismic survey spread bycoordinating the positioning of the first vessel control element, thesecond vessel control element, the first source control elementconfigured to be towed by the first vessel and the second source controlelement configured to be towed by the second vessel based on theestimated positions.
 41. The method of claim 1, wherein the positionsare estimated according to a spread model used to predict residuals, andfurther comprising: using the predicted residuals to estimate one ormore parameters of the spread model; and feeding the parameters backinto the spread model.
 42. The method of claim 1, wherein the collectedinput data includes one or more data types of pre-survey, operatorinput, present survey, near-real time, real-time survey, and simulatedsurvey.
 43. The method of claim 6, wherein the sensors measure currentvelocity, wind velocity or combinations thereof.
 44. A method,comprising: providing a seismic survey spread having one or morevessels, one or more source arrays and one or more spread controlelements, wherein the spread control elements comprise one or morevessel control elements, one or more source control elements and one ormore streamer control elements, wherein the source control elements andthe streamer control elements are configured to be towed by the one ormore vessels; and steering the source arrays in an inline motion usingthe vessel control elements; steering the source arrays in a cross linemotion using the source control elements; and coordinating thepositioning of the vessel control elements and the source controlelements and streamer control elements configured to be towed by the oneor more vessels.
 45. The method of claim 44, wherein the source controlelements comprises a winch system relative to one or more outer streamertow ropes.
 46. The method of claim 36, further comprising estimating oneor more positions of the spread control elements based on data receivedfrom one or more acoustic positioning receivers and one or morereference points on the seismic survey spread with respect to the earth,wherein the positioning of the vessel control elements, the sourcecontrol elements and the streamer control elements are coordinated basedon the estimated positions.
 47. The method of claim 36, wherein theseismic survey spread is controlled for reoccupying coordinates from aprior survey to achieve a 4D time-lapsed seismic survey.
 48. The methodof claim 39, further comprising estimating one or more positions of thespread control elements based on data received from one or more acousticpositioning receivers and one or more reference points on the seismicsurvey spread with respect to the earth, wherein the positioning of thestreamer control elements and the source control elements arecoordinated based on the estimated positions.
 49. The method of claim 1,wherein the sensors measure tension, vertical inclination, bodyorientation, acceleration or combinations thereof associated with thespread control elements.
 50. The method of claim 1, wherein the at leasttwo of the spread control elements further comprise a vessel controlelement.
 51. The method of claim 24, wherein the source control elementis coupled between a seismic source and the vessel.
 52. The method ofclaim 24, wherein the plurality of seismic survey spread elementscomprise one or more streamers.